UNIVERSITY OF GHANA 
  
 
 
 
 
STRUCTURAL AND PETROLOGICAL EVOLUTION OF THE SHAI 
HILLS TECTONIC SUTURE ZONE, SOUTHEASTERN GHANA. 
 
 
 
BY 
FOSU THOMAS 
(10279668) 
 
 
 
THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, 
LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR 
THE AWARD OF MPHIL GEOLOGY DEGREE. 
 
 
 
 
 
 
 
 
 
 
JULY, 2015. 
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DECLARATION 
This is to certify that this thesis is the result of research undertaken by Fosu Thomas towards 
the award of Master of Philosophy degree in Geology in the Department of Earth Science, 
University of Ghana, under the supervision of Prof. P.M. Nude and Dr. J.M. Kutu. 
 
……………………                                                         Date: ………………………………. 
Fosu Thomas  
(Student)              
                                                                           
 
……………………                                                          Date: ……………………………….. 
Prof. P.M. Nude 
(Supervisor)                                                                                   
 
 
 
……………………                                                          Date: ………………………………..  
Dr. J.M. Kutu 
(Co-Supervisor)                                                                                     
 
  
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ABSTRACT 
High Pressure (HP) mafic granulites (mafic garnet gneisses) in the Shai Hills suture zone 
occur as NE/SW trending isolated inselbergs in the Accra plains. They characterize the 
suture zone rocks of the Pan-African orogeny in Ghana. Various aspects of the Shai Hills 
suture rocks have been studied yet relatively little is known about its structural and 
deformational evolution. This work presents a new research into the structural, petrological 
and deformational evolution highlighting on metamorphism, petrogenesis and tectonic 
setting of the unique suture zone in Ghana by integrating field work, petrographic studies 
and whole rock major, minor and trace element (including REE) geochemical data. 
The high pressure granulites have undergone prograde metamorphism during subduction 
and followed by later retrograde metamorphism as a result of later exhumation. They record 
at least four distinct metamorphic episodes of which early prograde mineral assemblages are 
represented by mineral inclusions of hornblende + ilmenite within some porphyroblastic 
garnets and clinopyroxenes. The peak assemblage is characterized by granulite metamorphic 
assemblages of porphyroblastic garnet + clinopyroxene + quartz + plagioclase. The peak 
granulite stage was followed by three successive retrogressive facies stages during 
exhumation. The hornblende-granulite stage followed peak granulite stage and is 
characterized by the introduction of dark red hornblende and corona texture of 
clinopyroxene around garnet. This stage was then followed by amphibolite stage with the 
introduction of green hornblende rims of some clinopyroxene and garnet. The introduction 
of chlorite and epidote defines the greenschist metamorphic assemblage stage in the rock.  
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The terrane has undergone three main deformational stages: D1, D2 and D3. The D1 and D2 
are characterized by ductile deformation while the D3 is characterized by brittle deformation. 
Shear sense indicators in the terrane indicate an early dextral shearing and later sinistral 
shearing.  
Geochemical data suggest that, the high pressure granulitic rocks may have preserved the 
geochemical imprints of their magmatic basaltic protoliths where the rocks are mainly 
characterized by Island Arc Tholeiite (IAT) imprints with few with ocean crust (N-MORB) 
imprints. The HP granulite displays trace elements patterns similar to that of the mafic 
granulite lower crust (LC) composition. They show depletions in HREEs and have 
enrichment of LREEs. They are depleted in Cs, Rb and Th with enrichment in Ba and Sr. 
They also display averagely positive Eu anomaly which is similar to the lower crust (LC).  
The combination of petrography, mineral compositions, micro and macro structures and 
geochemical data suggest that the rocks may have undergone initial crustal thickening 
during subduction and collision related tectonic processes accompanied by prograde 
metamophism followed by exhumation, cooling and retrogression.  
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DEDICATION 
I dedicate this work to the Holy Trinity, my late father Mr. Philip Kojo Fosu and to my ever 
supporting and loving family.  
  
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ACKNOWLEDGEMENTS 
I thank the Almighty God for making it possible for me to undertake this project 
successfully. Throughout this project, the Lord has shown his mercies upon me and has 
provided me with good health that enabled me to complete the project successfully, thank 
you Lord.  
Secondly I would like to say a very big thank you to my supervisor, Prof. P.M Nude and my 
co-supervisor, Dr. J.M Kutu, and to the Dean of the School of Physical and Mathematical 
Science, University of Ghana, Prof. Daniel Asiedu and Emmanuel K. Abitty for providing 
me with the necessary assistance and taking time to shape and fine-tune this project, God 
bless you all.  
To the entire staff of Earth Science Department, University of Ghana (UG), I say thank you 
so much and may God bless you for the time and patience you had for me during this 
project. To my colleague Emmanuel Nyavor, I say thank you so much for helping me during 
the field work and to the Assembly man and the people of Dedenya for providing me 
accommodation and other support throughout my stay during the field work, God bless you 
all. To my friends, Edmund Yeboah, Manesseh Kodji Zodji, Asem Enoch and co., I say God 
bless you all for the various ways you contributed to the realization of the project, it has 
been a wonderful time having you around.  
Last but not the least, I would like to say thank you so much to my brother Theophilus Atta 
Fosu and to all my relatives for the many supports that you have offered me.  
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TABLE OF CONTENTS 
DECLARATION ..................................................................................................................... ii 
ABSTRACT ........................................................................................................................... iii 
DEDICATION ......................................................................................................................... v 
ACKNOWLEDGEMENTS .................................................................................................... vi 
LIST OF TABLES .................................................................................................................. xi 
LIST OF FIGURES ............................................................................................................... xii 
LIST OF ABBREVIATIONS.............................................................................................. xvii 
1.0 INTRODUCTION ....................................................................................................... 1 
1.1 Background .............................................................................................................. 1 
1.2 Objectives ................................................................................................................ 2 
1.3 Project Area ............................................................................................................. 3 
1.3.1 Accessibility ...................................................................................................... 6 
1.3.2 Physical Features ............................................................................................... 6 
1.3.3 Climate ............................................................................................................... 6 
1.3.4 Vegetation .......................................................................................................... 7 
1.3.5 Relief .................................................................................................................. 7 
1.3.6 Settlement, Population and Occupation ............................................................. 8 
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1.4 Structure of the Thesis ............................................................................................. 8 
2.0 REGIONAL GEOLOGY AND GEOLOGICAL SETTING .................................... 10 
2.1 Buem Structural Units ........................................................................................... 11 
2.2 Togo Structural Unit .............................................................................................. 12 
2.3 Dahomeyan Structural Unit ................................................................................... 13 
2.4 Previous Work ....................................................................................................... 17 
3.0 METHODOLOGY .................................................................................................... 20 
3.1 Desk Study and Literature Survey ......................................................................... 20 
3.2 Tools and Equipment ............................................................................................. 21 
3.3 Field Methods and Sampling ................................................................................. 21 
3.3.1 Reconnaissance Survey ................................................................................... 22 
3.3.2 Mapping Procedures ........................................................................................ 22 
3.4 Post Field Procedures ............................................................................................ 24 
3.4.1 Thin section Preparation .................................................................................. 25 
3.4.2 Geochemical Analysis ..................................................................................... 26 
3.4.3 Map Digitization, Stereographic projections and Cross Section Drawing. ..... 27 
4.0 RESULTS .................................................................................................................. 28 
4.1 Field Relation and Petrography ............................................................................. 28 
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4.1.1 Mafic Garnet Gneisses ..................................................................................... 30 
4.1.2 Pyroxene Bearing Amphibolite ....................................................................... 47 
4.1.3 Pyroxenite ........................................................................................................ 51 
4.1.4 Fault Breccia .................................................................................................... 54 
4.2 Deformations, Structures and Kinematics ............................................................. 56 
4.2.1 Gneissic Bands and Foliations. ........................................................................ 56 
4.2.2 Mineral lineation. ............................................................................................. 60 
4.2.3 Ptygmatic Folds ............................................................................................... 64 
4.2.4 Faults................................................................................................................ 66 
4.2.5 Joints ................................................................................................................ 70 
4.2.6 Boudins ............................................................................................................ 70 
4.3 Major and Trace Element Geochemistry ............................................................... 72 
4.3.1 Major Element Geochemistry .......................................................................... 72 
4.3.2 Trace Element Geochemistry .......................................................................... 78 
5.0 DISCUSSIONS.......................................................................................................... 91 
5.1 Field Relation and Petrography ............................................................................. 94 
5.2 Metamorphism ..................................................................................................... 100 
5.3 Deformations, Structures and Kinematics ........................................................... 101 
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5.3.1 D1 Deformation .............................................................................................. 102 
5.3.2 D2 Deformation .............................................................................................. 103 
5.3.3 D3 Deformation .............................................................................................. 104 
5.4 Geochemistry ....................................................................................................... 106 
5.4.1 Classification ................................................................................................. 106 
5.4.2 Tectonic Setting ............................................................................................. 109 
5.4.3 Petrogenesis ................................................................................................... 116 
6.0 CONCLUSIONS AND RECOMMENDATIONS .................................................. 125 
REFERENCES .................................................................................................................... 130 
APPENDIX.......................................................................................................................... 144 
 
  
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LIST OF TABLES 
Table 1:  Average modal composition (vol %) of minerals in the garnet hornblende gneiss.45 
Table 2: Average modal composition (vol%) of minerals in the pyroxene garnet gneiss. .... 47 
Table 3: Average modal composition (vol %) of minerals in pyroxene bearing Amphibolite.
 ................................................................................................................................... 51 
Table 4: Average modal composition (vol%) of minerals in pyroxenite. ............................. 54 
Table 5: Average modal composition (vol%) of minerals in fault breccia. ........................... 55 
Table 6:  Structural attitude (degree) of foliation planes. ...................................................... 58 
Table 7: Structural attitude (degree) of mineral lineation. .................................................... 61 
Table 8: Structural attitude (degree) of Ptygmatic fold axis. ................................................. 64 
Table 9: Structural attitude (degree) of faults planes in the area. .......................................... 67 
Table 10: Major and trace elements composition of the pyroxene garnet gneiss. ................. 75 
Table 11: Major and trace elements composition of garnet hornblende gneiss. ................... 76 
Table 12: Major and trace elements composition. ................................................................. 77 
  
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LIST OF FIGURES 
Figure 1.1: Geological map of Ghana with the study area shown by the black rectangle. 
(Compiled from Bates (1995), Hasting (1982), Davis, et. al (1994)) .......................... 4 
Figure 1.2: Topographical Map of the study area. ................................................................... 5 
Figure 2.1: Tectonic map of the Dahomeyides of southeastern Ghana (after Attoh, 1998)   
showing the location of the study area. ..................................................................... 16 
Figure 4.1: Outcrop map of the study Area. .......................................................................... 29 
Figure 4.2: Field photographs showing (a) subhorizontal layers of mafic and felsic bands in 
mafic garnet gneiss; at Eastern Quarries, Mampong Hill (b) Compositional felsic 
bands in the mafic garnet gneiss at Mampong Hill (c) Porphyroblastic garnet of 
different sizes at PW Quarries, Naglayo Hills (d) Alternating bands of mafic, felsic 
and garnet bands in the mafic garnet gneiss at the West Coast Quarries, Naglayo 
Hills. ........................................................................................................................... 31 
Figure 4.3: Field Photographs showing (a) compositional bands with stretched hornblendes 
(b) Compositional bands folded into ptygmatic fold (c) Ptygmatic folds (d) Pressure 
Shadow formed at the edges of porphyroblastic garnet minerals. ............................. 34 
Figure 4.4: Field photographs showing (a) stretched pyroxenite within the mafic garnet 
gneisses at the Shai Hill Game Reserve (b) brittle fault at the PW Quarries; Naglayo 
Hill (c) brittle fault zone observed at PW Quarries at Naglayo Hill (d) Joints found at 
the Shai Hill game reserve. ........................................................................................ 35 
Figure 4.5: (a) Photomicrograph showing garnet minerals. Plane polarized light (Ppl) (b) 
Photomicrograph showing garnet and clinopyroxene porphyroblast (Ppl) (c) Garnet 
porhyroblast with hornblende inclusion (Ppl) (d) Poikiloblastic texture of garnet with 
inclusions of quartz and opaque minerals (Ppl). Garnet (Grt), Plagioclase (Pl), 
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Microcline (Mc), Pyroxene (Px), Clinopyroxene (Cpx), Orthopyroxene (Opx), 
Hornblende (Hbl), Quartz (Qtz), Chlorite (Chl), Opaque mineral (Op). ................... 40 
Figure 4.6: (a) Photomicrograph showing Clinopyroxene porphyroblast with rims of chlorite 
with the S1 foliation around it. (Ppl). (b) Photomicrograph showing Clinopyroxene 
porphyroblast with rims of chlorite. (Xpl).  (c) Hornblende rims around broken 
Clinopyroxene (Ppl) (d) Hornblende rims around broken Clinopyroxene (Xpl). Same 
Abbreviations are used. ............................................................................................. 41 
Figure 4.7: (a) and (b) Photomicrographs are showing porphyroblastic Clinopyroxene with 
rims of hornblende intracrystalline fractures. (Ppl) (c) and (e) Broken and cracked 
Clinopyroxene porphyroblast (Ppl) (d) and (f) Broken Clinopyroxene porphyroblast. 
(Xpl). Same Abbreviations are used. ......................................................................... 42 
Figure 4.8: (a) Photomicrogragh showing deformed pre kinematic garnet porphyroblast with 
rims of hornblende and S1 foliation wrapping around it. (Ppl). (b) Syn kinematic 
garnet with inclusion of Clinopyroxenes. (Xpl). (c) Garnet with rutile exsolution 
(Xpl). (d) Syn kinematic garnets (Ppl). (e) Garnet porphroblast with inclusions of 
quartz and plagioclase (Xpl).  (f) Corona texture of Clinopyroxenes and quartz 
around garnet porphyroblast (Ppl). Same Abbreviations are used. ........................... 43 
Figure 4.9: (a) Photomicrograph showing L1 mineral lineation in the S1 foliation. (b) 
Photomicrograph showing deformed polysynthetic twining in plagioclase and 
deformed quartz. (Ppl). Same Abbreviations are used. ............................................. 44 
Figure 4.10: Photomicrograph showing garnet hornblende gneiss. (Xpl) Same Abbreviations 
are used. ..................................................................................................................... 45 
Figure 4.11: Photomicrographs of pyroxene garnet gneiss. (Xpl). Same Abbreviations are 
used. ........................................................................................................................... 46 
Figure 4.12: Field photograph of pyroxene bearing amphibolite. ......................................... 48 
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Figure 4.13: Field photograph of amphibolite tectonically welded into host mafic garnet 
gneiss at Eastern quarries, Mampong Hill. (b) Gradational contact between the 
amphibolite and the mafic garnet garnet gneiss. ....................................................... 48 
Figure 4.14: Photomicrographs of pyroxene bearing amphibolite. (Ppl). Same Abbreviations 
are used. ..................................................................................................................... 50 
Figure 4.15: Field photograph showing pyroxenite thrusted into the mafic garnet gneiss at 
Eastern Quarries. Mampong Hill. .............................................................................. 52 
Figure 4.16: Field photograph showing pyroxenites boudins in the mafic garnet gneiss at 
PW Quarries. Naglayo hill. ........................................................................................ 52 
Figure 4.17: (a) and (c) Photomicrographs showing Clinopyroxene (Cpx) and 
Orthopyroxene (Opx) in the pyroxenite. (Ppl). (b) and (d) Photomicrographs 
showing Clinopyroxene (Cpx) and Orthopyroxene (Opx) in the pyroxenite. (Xpl). 
Same Abbreviations are used. .................................................................................... 53 
Figure 4.18: Photomicrograph of the Fault Breccia. (Ppl). Same Abbreviations are used. ... 55 
Figure 4.19: Field photographs showing gneissic banding and foliations in the mafic garnet 
gneisses. Shai hills. .................................................................................................... 57 
Figure 4.20: (a) Poles plot of foliations in mafic garnet gneisses (b) Poles plot with plane to 
poles plot in the mafic garnet gneiss.......................................................................... 59 
Figure 4.21: Contoured poles-density plot of foliations in mafic garnet gneisses. ............... 59 
Figure 4.22: Stretched hornblende within compositional banding in the mafic garnet gneiss.
 ................................................................................................................................... 60 
Figure 4.23: Shows Poles plot of lineations in mafic garnet gneisses. .................................. 62 
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Figure 4.24: Photograph showing dextral sense of shearing by sheared pyroxenite boudin in 
the mafic garnet gneiss during D1 deformation at Shai Forest reserve...................... 63 
Figure 4.25: Photograph showing dextral sense by clockwise rotation of garnet in the mafic 
garnet gneiss during D1 deformation at PW Quarries, Naglayo Hill. ........................ 63 
Figure 4.26: Poles plot of fold axis of ptymatic folds. .......................................................... 65 
Figure 4.27: Field photograph showing normal faults in the mafic garnet gneiss at PW 
Quarries, Mampong Hill. ........................................................................................... 66 
Figure 4.28: Poles plot of faults in the mafic garnet gneisses. .............................................. 68 
Figure 4.29: Photograph showing overall sinistral sense of shear during the D3     
deformations. ............................................................................................................. 69 
Figure 4.30: Structural map of the study area. ....................................................................... 71 
Figure 4.31: Chondrite-normalised plots of the REE composition of the rocks in the study 
area (a) Pyroxene garnet gneisses. (b) Garnet hornblende gneiss. With lower crust 
(LC) and Normal MORB (N-MORB). ...................................................................... 81 
Figure 4.32: Chondrite-normalised plots of the REE composition of the rocks in the study 
area (c) Pyroxene bearing amphibolite (d) fault breccia and pyroxenite. With lower 
crust (LC) and Normal MORB (N-MORB). ............................................................. 82 
Figure 4.33: Chondrite normalized plots of the incompatible trace element composition of 
the rock in the study area. (A) Pyroxene garnet gneisses (B) Garnet hornblende 
gneisses ...................................................................................................................... 84 
Figure 4.34: Chondrite normalized plots of the incompatible trace element composition of 
the rock in the study area. (C) Pyroxene bearing amphibolite (D) Fault breccia (E) 
Pyroxenite. ................................................................................................................. 85 
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Figure 4.35: N MORB normalized plots of the incompatible trace element composition of 
the mafic garnet gneisses. (A) Pyroxene Garnet Gneiss (B) Garnet Hornblende 
Gneiss. ....................................................................................................................... 87 
Figure 4.36: Chondrite normalized plots of transition element composition of the rocks in 
the study area. (A) Pyroxene garnet gneiss (B) Garnet hornblende gneiss. .............. 89 
Figure 4.37: Chondrite normalized plots of transition element composition of the rocks in 
the study area. (C) Pyroxene bearing amphibolite (D) Fault breccia (E) Pyroxenite. 90 
Figure 5.1: Geological map of the area. ................................................................................ 92 
Figure 5.2: Cross section of the Study area. .......................................................................... 93 
Figure 5.3: Geochemical classifications of rocks in the study area using TAS diagram (a) Le 
Bas et al., (1986) (b) Cox et al., (1979). .................................................................. 107 
Figure 5.4: Classification plot of SiO2 Versus K2O (after Peccerillo and Taylor, 1974). ... 108 
Figure 5.5: Tectonic discrimination diagram Cr vs Y plot (Fields for MORB and IAT after 
Pearce et al., 1984) ................................................................................................... 110 
Figure 5.6: Tectonic discrimination diagram of Ti vs Zr plot (Pearce and Can, 1973) ....... 111 
Figure 5.7: Tectonic discrimination diagram of MgO-FeO-Al2O3 (Pearce et al., 1977). .. 112 
Figure 5.8: Tectonic discrimination diagram of MnO2, TiO2 and P2O5 (Mullen, 1983)...... 113 
Figure 5.9: AFM diagram showing samples from the study area (After Irvine and Barager, 
1971). ....................................................................................................................... 118 
Figure 5.10: Proposed model for the Pan African suture zone in Ghana based on Wilson 
cycle reconstructions. (Modified after Attoh and Nude, 2008). .............................. 124 
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LIST OF ABBREVIATIONS 
WAC - West African Craton 
SE - South East 
NW - North West 
NE - North East 
ENE - East North East 
WSW - West South West 
NNW - North North West 
SSE - South South East 
HIPGE - High Pressure Granulites and Eclogites 
UHP - Ultra High Pressure 
HP - High Pressure 
TSB - Trans Saharan Belt 
Ma - Million Years 
Ga - Billion Years 
GPa - Giga Pascal 
REE - Rare Earth Element 
HREE - Heavy Rare Earth Element 
LREE - Light Rare Earth Element 
MREE - Middle Rare Earth Element 
HFSE – High Field Strength Element 
LILE – Large Ion Lithophile Element 
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ICP-AES - Inductively Coupled Plasma Atomic Emission Spectrometry 
ICP-MS - Inductively Coupled Plasma Mass Spectrometry  
Fig - Figure 
PPL - Plane Polarized Light 
XPL - Cross Polarized Light 
CIPW - Cross Iddings Pirsson-Washington 
Wt.% - Weight Percentage 
Ppm - Part per million 
Norm - Normative 
MORB - Mid Ocean Ridge Basalt 
E-MORB - Enriched Mid Ocean Ridge Basalt 
N-MORB - Depleted Mid Ocean Ridge Basalt 
LC - Lower Crust 
IAT- Island Arc Tholeiite 
TAS - Total Alkalis and Silica  
CAB - Continental Alkaline Basalt 
OIA - Ocean Island Alkali Basalt 
Mg# - Magnesium Number   
GPS - Global Positioning System   
Grt – Garnet  
Pl – Plagioclase  
Mc – Microcline  
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Px – Pyroxene  
Cpx – Clinopyroxene 
Opx – Orthopyroxene 
Hbl – Hornblende  
Qtz – Quartz 
Chl – Chlorite 
Op – Opaque mineral 
 
  
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CHAPTER ONE 
1.0 INTRODUCTION 
1.1 Background 
In the Pan-African Dahomeyide orogenic belt, the granulitic, eclogitic or mafic to ultramafic 
Dérouvarou (Benin), Kabye-Kpaza, Djabatouré-Anié, Agou-Ahito (Togo) and Akuse or 
Shai (Ghana) Hills constitute series of mountains consisting the suture zone (Affaton and 
Tairou, 2012). The Pan-African Dahomeyide orogen of western Africa represents a model of 
collision belt between the passive continental margin of the West African craton (WAC) and 
a group of eastern plates, the Benino-Nigerian shields (Affaton et al., 1980). This orogen in 
West Africa is the southern extension of the 2000 km long Trans-Saharan belt which formed 
on the eastern margin of the West African Craton (Attoh et al., 2007). 
The Pan-African belt has been subdivided into three main units (Affaton et al., 1980): the 
western external units, thus the deformed edge of the West African craton (WAC) with its 
cover rocks consisting of craton verging nappes and thrust sheets bounded by ductile shear 
zones; Buem and Atacora structural units, the eastern internal units, exotic rocks that form 
the granitoid gneiss complexes east of the suture zone and thirdly, the suture zone. 
Numerous ultrabasic and basic bodies of various sizes are scattered throughout the suture 
zone and display contrasted lithological and metamorphic features (Me´not, 1980). Along 
this suture zone, high-pressure granulites and eclogites (HIPGE) of basaltic composition are 
the dominant rock types (Attoh, 1998a; Agbossoumonde et al., 2001; Attoh and Morgan, 
2004). There are some amphibolite rocks in association with the high-pressure granulites 
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and eclogites (HIPGE). The distinctive mafic and ultramafic rocks in the suture can be 
traced more or less continuously for about 1000 km and are referred to as the Shai Hills 
Gneisses in southeastern Ghana (Attoh et al., 1997), Kabye Complex in northern Togo and 
rocks of the Agou Complex in southwestern Togo (Sylvain et al., 1986; Agbossoumonde, 
1998). 
Various petrological and geochronological investigations have been conducted on the highly 
metamorphosed and deformed rocks of the Shai Hill suture. However, the Dahomeyide 
suture zone located in Ghana (Shai Hills) have received relatively little attention, especially 
on its structural and deformational evolution compared to other Pan African suture zones 
rocks. Most studies in the area do not sufficiently lay emphasis on the structural and time 
relation between deformations. This research intends to contribute to the structural 
characterization and provide detailed sequential deformational events characterizing the Shai 
hills suture zone rocks. 
1.2 Objectives 
The objectives of this research are to;  
1. determine the structural and deformational evolution of the rock units of the suture 
zone. 
2. determine the paragenesis of the main rock facies. 
3. establish the time relation between deformations and structures in the area. 
4. deduce field relations of the rocks in the study area; produce a structural map and a 
composite geological map with cross section of the area. 
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The key outcome of this research program is the documentation of regular structural 
geometry at macro to micro scale and the interpretation of the distribution of the structures 
throughout the study area and timing of deformational events in the area which have not 
been dealt with sufficiently.  
To achieve these objectives, first, the geographical and the geological settings of the Shai 
Hill suture zone rocks are described relative to the regional context.  This is followed by a 
stepwise presentation of results from this study. The results and their interpretations are 
treated in detail to finally arrive at conclusions, where a geodynamic model for the Shai 
Hills suture zone is proposed. 
1.3 Project Area 
The Shai Hills area is located in the Dangme West District of the Greater Accra Region 
Ghana. It is bounded by the coordinates (0o01’E, 5o 56’ N) (0o 06’E, 5o 56’N), (0o 01’E, 5o 
50’N) and (0o, 06’E, 5o 50’N). It is within the field sheet number 0500A1. It has an 
approximate area of about 60km2. The Shai Hills area is a hub of several stone quarries 
which generate huge revenue for Ghana through taxes.  
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Figure 1.1: Geological map of Ghana with the study area shown by the black rectangle. 
(Compiled from Bates (1995), Hasting (1982), Davis, et. al (1994)) 
 
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Figure 1.2: Topographical Map of the study area. 
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1.3.1 Accessibility 
The study area can be accessed by road through the Tema – Akosombo highway. Notable 
towns are Dzopaanya, Dedenya, Mampong and Kissehkope and surrounding areas. The 
Tema to Ho road is a first class road which passes through the study area at the 
Southwestern corner through the study area to the northeastern part. It has also other roads 
footpaths leading to various villages in the study area.  Through the footpaths and roads, the 
field mapping exercise was successful. 
1.3.2 Physical Features 
The study area is a SW-NE trending range of hills and inselbergs which are the most 
obvious physical feature on the base map. The Shai Hills vary in width from 65 to 95 km. 
Waterways and dams are also conspicuous physical features in the area. The waterways flow 
in the valleys between the rocks and join others to form dams. 
1.3.3 Climate 
The study falls within the wet semi equatorial climatic region (Dickson and Benneh, 1988). 
The mean annual rainfall is between 125 and 200 cm. The rainfall pattern of the study area 
is of a double maxima type where two rainy seasons occur in a year. The first rainy season is 
from May to June and the second rainy season is from September to October. Heaviest 
rainfall mainly occurs in June. The mean annual temperature is 26.6oC (Dickson and 
Benneh, 1988). Average relative humidity is about (75-80%) during the two rainy seasons 
and the lowest (70-80%) during the rest of the year (Dickson and Benneh, 1988). 
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1.3.4 Vegetation 
The area of study has a low cover of vegetation cover but due to the Shai Hills forest reserve 
there is a quiet dense forest which is kept as a resource. The areas east of the Shai Hills and 
the north eastern part of the field of study have very low and thin vegetation which could be 
due to clayey nature of soil and are referred to as the lowlands. Some of the highland areas 
have thick and impenetrable vegetation. Vegetation, where it is present, forms an important 
part of the physical environment and helps greatly in the definition of the recourses and 
character of the area (Dickson and Benneh, 1988). The forest areas are mainly highlands and 
exhibit semi deciduous forest type during the long dry season from November to March. The 
lowland is mainly the Guinea savannah (Dickson and Benneh, 1988). They are covered by 
low bushes and open grassland (Dickson and Benneh, 1988). The principal soil is the forest 
ochrosols at the highlands. A soil that is highly coloured soil from highly weathered parent 
mafic or felsic materials. The soil at the lowlands is the lateritic sandy soil. The nature of the 
soils impedes downward drainage and causes waterlogging during the wet season (Dickson 
and Benneh, 1988). These hills are poorly covered with soil, and their slopes are mostly 
covered with gneiss debris. The area is covered with dark brownish-black silt and clayey soil 
(Mani, 1977). 
1.3.5 Relief 
The area has a relief system with the highlands running gently into the lowlands. Some of 
the low areas are wide valleys which are characterized by waterways and dams. The 
highlands run as stretches of hills from the NE-SW with the highest peak around 290m 
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above sea level. The highlands mostly occur as ridges and a few isolated hills. They mostly 
consist of gneissic rocks. The lowlands on the other hand occur just east of the area of study 
then downwards towards the south with the lowest altitude around 50m above sea level. The 
lowlands have relatively flat topography with quite a number of waterways running across in 
different directions but most of them are dried up. 
1.3.6 Settlement, Population and Occupation  
The study area is sparsely populated when compared to urban cities. Major towns and 
communities include Doryumu, Dzopaanya, Dedenya and Mampong. The people of these 
communities speak the Krobo language and Ga-Adangme. There also some Fulanis 
herdsmen who live in the study areas with a main occupation of cattle farming. The people 
in the semi-urban areas live in modern semi-detached homes while the people in the smaller 
communities live in mud houses and wooden buildings. The major occupations of the people 
in these communities are farming, cattle rearing and trading. Stone quarrying is also an 
occupation of the inhabitants. 
1.4 Structure of the Thesis 
The Thesis work has six (6) chapters with each chapter addressing a main heading. Chapter 
one introduces the field of research. It also deals with the location, and socioeconomic 
activities of the research area. The objectives, importance and the problems the research 
seeks to address are also mentioned. Chapter two gives a general overview of the national 
and local geological setting of the study area. This chapter also deals with the major types of 
formations found in Ghana. Chapter three deals with the materials and the methods used in 
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collecting and analyzing data. Chapter four and five presents the results obtained and 
discusses the results presented in the various maps obtained. Chapter six presents 
conclusions from the study. 
 
  
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CHAPTER TWO 
2.0 REGIONAL GEOLOGY AND GEOLOGICAL SETTING 
The assembly of NW Gondwana from various cratonic fragments postulated to be derived 
from the breakup of Rodinia supercontinent (Hoffman 1991) resulted in Pan-African 
(Neoproterozoic) orogens including the 2000 km long Trans-Saharan orogen (Caby 1987; 
Trompette, 1994) located on the eastern margin of the West African craton (WAC). The 
southeastern segment of the Trans-Saharan belt exposed in southeastern Ghana and 
adjoining parts of Togo and Benin comprises the Dahomeyide orogen (Affaton et al.1991; 
Castaing et al. 1993; Attoh et al. 1997). 
According to Attoh et al. (1997), the principal tectonic elements of the Dahomeyide orogen 
are: 
(1) the deformed edge of the West African craton (WAC) with its cover rocks consisting of 
craton verging nappes and thrust sheets bounded by ductile shear zones that is the external 
units made of up the Buem structural unit, The Togo structural unit and the Kara gneisses. 
(2) the suture zone representing the eastern boundary of the autochthonous West African 
Craton (WAC). 
(3) exotic rocks that form the granitoid gneiss complexes east of the suture zone. That is the 
internal units. 
The Pan-African Dahomeyide orogen in West Africa is interpreted to have formed by the 
collision of exotic blocks with the passive continental margin of the West African craton 
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(Caby, 1987; Affaton et al., 1991; Agbossoumonde et al., 2004; Attoh and Morgan, 2004). 
The western zone is the external nappes domain, which corresponds to the Buem and 
Togo/Atacora monocyclic metasedimentary units. The eastern zone is the internal units 
nappes domain that formed the composite basement of the Benin Nigeria Shield (Nude et 
al., 2009). Intermediate nappes domain is a narrow and structurally complicated zone called 
the Suture zone which is made up of ultramafic to mafic rocks together with high grade 
metasediment (Nude et al., 2009). 
The terrane is a convergence tectonic closure and sutured collision boundary zone that 
occurs along the easten margin of the West African Craton. It trends inland northeast-wards 
from the Gulf of Guinea through southeastern Ghana, the Republics of Togo and Burkina 
Faso to Mali (Kutu et al., 2014). 
2.1 Buem Structural Units 
The Buem consists of two lithologic assemblages, volcanic and sedimentary. The volcanic 
assemblage is made up of pillow basalt, agglomerate, hawaiite and trachyte. The 
sedimentary assemblage, which encloses the volcanics, consists of red shales, feldspathic to 
quartz arenite, conglomerate, tillite, jasper and minor limestone. Whereas the volcanics were 
deposited in a submarine environment, the sediments appear to be shallow water to subaerial 
in origin (Jones, 1990). Kesse (1985) states that the rocks are strongly folded whiles Jones 
(1990) describes the Buem as an eastward dipping homoclinal sequence. 
 
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2.2 Togo Structural Unit 
The Togo Structural Unit is found to the east of the Buem unit. The north east trending Togo 
or Akwapimian hills trend from west of Accra to the Republic of Togo and Benin. 
According to Kesse (1985), Togo unit can be divided into two: 
I. Togo units of the main Akwapim, Awudome-Amedzofe range.  
II. Togo units with Dahomeyan on both margins.  
The main Akwapim, Awudome-Amedzofe range further consists of: 
I. interbedded schists and phyllites, usually sericitic or chlorite schist making the upper 
schist.  
II. massive quartzites, chert, quartz schists forming the upper quartzite units.  
III. interbedded schists and phyllites usually sericitic or chloritic, locally conglomerate 
forming the lower schist. 
The western margin of the Akwapimian range consist of quartz schists, quartz sericite schist 
and locally massive quartzites. Togo units are being surrounded by Dahomeyan units. These 
are eastern scarps and considered as outliers. They are found in Abutia, Kabakaba, Kluma 
hills, Ziavi and those north east up to Kpedze. They are made up of phyllites, flaggy and 
cataclastic quartzites marked by folding, cross faulting and shearing and generally well 
foliated. Quartzites are dominant in the south but phyllites are prominent towards the north 
(Kesse, 1985). They display penetrative structures in the quartzites and quartz mica schists 
with prominent lineation typified by fold rocks. Recumbent folds with rotated hinges to the 
north and south south-east indicates transport directions (Kesse, 1985). 
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The geology of the Togo unit has been described by Kitson (1928) and Junner (1940). The 
main lithologies within the Togo unit are quartzites, schists, phyllites and phyllonites. Minor 
amounts of shale and siliceous limestone occur within the Togo unit. Often rocks in the 
thrust zones are strongly sheared resulting in brecciation as well as flattening of quartz 
grains and mylonitization. 
Generally, the quartzites and the phyllites within the Togo unit are intensely deformed, and 
mostly occur as craton verging recumbent folds with regionally pervasive sub-horizontal 
foliation. Regarding the age of the Togo unit, Attoh et al. (1997) reported 40Ar/39Ar dates 
from muscovite in the quartzites in Ghana to be 579.4 ± 0.8 Ma.  
2.3 Dahomeyan Structural Unit  
The Dahomeyan structural unit of southeastern Ghana belongs to the Pan-African terrain, 
also called the Dahomeyides (Bessoles and Trompette, 1980; Attoh, 1990). The geology of 
the area has been described by Holm (1974), and Kesse (1985). These workers described the 
Dahomeyan in Ghana as consisting generally of two belts of felsic gneiss that alternate with 
two belts of mafic gneiss. These belts are up to 30 km wide and can be traced from Fete in 
the west of Accra to Agu in the Republic of Togo and beyond. The regional strike is 
northeast with moderate dips to the southeast. Others such as Grant (1969) and Holm (1974) 
considered the Dahomeyan as a reworked Archean basement of the late Precambrian Pan-
African orogen and having records of polycyclic deformation.  
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Recent works by Attoh (1990, 1998); Attoh et al. (1991, and 1997) and Agbossoumonde et 
al. (2004) consider the Dahomeyan structural unit as consisting of various structural units 
based on age and tectonics. The principal litho-tectonic units identified are: 
(i) quartzo-feldspathic and augen-gneisses referred to locally as the Ho gneisses 
(ii) a suture zone of distinct mafic and ultramafic rocks, and  
(iii) a granitoid gneiss-migmatite assemblages east of the suture zone. 
The Ho gneisses represent the deformed edge of the West African Craton and includes 
augen gneisses and mylonitic gneisses most of which are hornblende-rich and often sheared 
and at some localities (Attoh et al., 1996). Doleritic and dioritic intrusives are common 
along contact zones with the Togo cover rocks and may represent late stage magmatism 
associated with the Pan- African orogen. The Ho gneiss recorded ages of 2176 ± 44 Ma on 
whole rock Rb-Sr isochron (Agyei et al., 1986). 
The suture zone, east of the Ho gneisses and marks the boundary between the autochthonous 
West African Craton and the exotic rocks of the Dahomeyan lithotectonic units. Rocks of 
the suture zone are high pressure granulite and eclogite facies assemblages of garnet-
hornblende-pyroxene gneiss referred to locally as Shai-Hills gneiss, in addition to 
pyroxenites and metanorites (Attoh and Nude 2008). U-Pb analyses of zircon from the high 
pressure granulites gave an age of 610 ± 2 Ma (Attoh et al., 1991) which was interpreted as 
the time of peak granulite metamorphism. In southeastern Ghana, the HP mafic granulites 
have been referred to as Shai Hills gneiss (Attoh et al., 1997), and are tectonically 
juxtaposed with the alkaline gneiss complex in the suture zone (Attoh and Nude 2008). 
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Hirdes and Davis (2002) reported U-Pb analyses of zircon from the same high pressure 
garnet granulite unit, the youngest age recorded their analysis is 603 ± 5 Ma which is close 
to the time of peak metamorphism. Similar age of 613 ± 1 Ma from zircon Evaporation 
(207Pb/206Pb) was published by Affaton et al. (2000) for the suture zone rocks in northern 
Togo. Hornblende separates from the mafic granulites yielded 40Ar/39Ar ages between 587 
and 567 Ma, interpreted as the time of exhumation of the nappes (Attoh et al., 1997). Taken 
together, these ages confirm high pressure metamorphism of the suture zone rocks around 
603-613 Ma and exhumation through the hornblende ages around 580-570 Ma (Attoh et al., 
2007). 
To the coterminous east of the Shai Hills and in tectonic contact with the suture zone rocks 
are felsic gneisses and migmatites (Attoh et al., 1996). K-Ar dates from biotite in the 
migmatite gave an age of 452-506 Ma (Agyei et al., 1986). It thus appears that 
migmatization in this area may have been associated with the late stages of the Pan-African 
orogen.  
 
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Figure 2.1: Tectonic map of the Dahomeyides of southeastern Ghana (after Attoh, 1998)   
showing the location of the study area. 
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2.4 Previous Work 
Previous work has been done extensively by Knorring and Kennedy (1958), Attoh and 
Morgan (2004), Attoh and Nude (2008) and Kutu et. al., (2014).  According to Attoh and 
Nude (2008), the association of carbonatite and ultrahigh-pressure (UHP) metamorphic 
rocks in the Dahomeyide suture zone of southeastern Ghana is unique among the 
Neoproterozoic orogens that surround the West African craton (WAC). Carbonatite occurs 
in an alkaline complex that decorates the sole thrust of the suture zone and is characterized 
by high concentrations of incompatible trace elements such as light rare earth elements 
(LREE), Sr and Ba.  
Within the suture zone deformed alkaline rocks, including carbonatites, together with mafic 
granulites form an imbricate stack of thrust panels that involve 2.1 Ga rocks of the West 
African Craton (WAC) basement (Attoh and Nude, 2008). The dominant rocks units of the 
suture zone are mafic granulites in which garnet megacrysts preserve a diagnostic 
microstructure of Ultra High Pressure (UHP) metamorphism; consisting of a 
crystallographically controlled array of exsolved rutile rods in garnet. Metamorphic 
Pressures estimated from Ti concentrations in the inferred precursor garnet indicate pressure 
greater than 3 GPa, which requires subduction (and exhumation) of the suture zone rocks to 
and from mantle depths during collisional orogeny on the West African Craton (WAC) 
margin. Available age constraints on carbonatite magmatism suggest that continental rifting, 
leading to the formation of the passive West African Craton (WAC) margin c. 700 Ma, 
occurred c. 100 Ma before intrusion of carbonatite, which was preceded by HP and UHP 
metamorphism at 610+5 Ma (Attoh and Nude, 2008). 
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According to Kutu et al. (2014), the two most abundant and prominent rocks present in the 
suture zone are the mafic garnet gneiss and amphibolite. These rocks are associated with 
minor occurrences of dolerite, nepheline gneiss, carbonitite, alkali gneiss, quartzite and 
quartz muscovite schist with metamorphic grade ranging from amphibolite to granulite 
facies. The mafic garnet gneisses are strongly sheared deformed and recrystallized with 
mafic garnet gneiss doubly metamorphosed, well consolidated and competent (Kutu et al., 
2014).  
According to Kutu et. al. (2014), the amphibolite is characterized by tremolite actinolite 
amphiboles, plagioclase feldspar and minor quartz and iron oxides. The garnet gneiss is also 
characterized by quartz, feldspars, hornblende, hypersthene-dominated pyroxenes, pyrope-
almandine garnets some with rutile inclusions, minor micas and scapolite. Deformation of 
the terrane and rocks was intense, and by compressional thrusting and shearing. The 
direction of tectonic transport from southeast to northwest in a craton-ward vergence on 
subparallel thrust planes (Kutu et al., 2014). 
The amphibolite and garnet gneiss have undergone temperature and pressure changes in the 
solid state, and transformed from the parent igneous basaltic protolith into the 
metamorphosed and deformed amphibolite and polymetamorphosed garnet gneiss 
respectively. The metamorphic grade ranges from amphibolite to granulite facies. The 
terrane has undergone at least two phases of metamorphism, from the amphibolite facies 
change from basalt to amphibolite and granulite facies change from the amphibolite to the 
hornblende-garnet gneiss (Kutu et al, 2014). 
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According to Tairou and Affaton (2012), the suture zone includes rock types indicating Pan-
African crustal thickening and comprising various granulites, pyroxenites and eclogitoids, 
associated with their heteromorphic equivalents and rare chromitites and metasediments. 
The Pan-African tectogenesis is composed of five phases defined as Dn, Dn + 1, Dn + 2, Dn 
+ 3 and Dn + 4 (Tairou and Affaton, 2012).  The Dn phase corresponds to the long process 
culminating in the collision between the active eastern margin, or the Benino-Nigerian plate, 
and the passive Western margin belonging to the West-African Craton. This collisional 
episode is materialized by a Sn foliation that is obliterated by the subsequent phases in the 
polycyclic basement and suture zone nappes with the Dn + 1 phase represents the main 
episode in Dahomeyide structuring leading to nappe and slice individualization and their 
west stacking. This tangential phase is materialized by the Sn + 1 regional foliation and by 
mineral or stretching Ln + 1 lineation. The Dn + 2 to Dn + 4 post nappe episodes 
successively rework the Sn + 1 foliation. They resulted in Pn + 2 folds, with submeridian to 
NE-SW axes, Pn + 3 antiforms and synforms, with NE-SW axes, and Pn + 4 virgations, with 
NE-SW to ENE-WSW axes (Tairou and Affaton, 2012). 
The overall structure of the suture zone is interpreted to have resulted from early east–west 
compression, which produced the north–south imbricate thrust slices followed by NNW-
directed thrusting in the orogen (Attoh et al., 1997).  
  
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CHAPTER THREE 
3.0 METHODOLOGY 
The research was carried out in three main stages were employed: The Pre field stage, field 
stage and then post field stage. Samples were taken from rock exposures and from outcrops 
for petrographic and structural studies, thin sections prepared at the Earth Science 
Department, University of Ghana were examined using a Petrological microscope. 
3.1 Desk Study and Literature Survey 
The base map of the study area was first acquired and the coordinates were located on the 
geological map of Ghana to know the precise geological terrene which it is located. A 
literature searches to know the formations in the study area which included information on 
petrology, mineralogy, structures and field relationship and general geology standard text 
was carried out. This provided the necessary background knowledge that would facilitate 
field investigation. Literature review actually lasted throughout the whole period of this 
research. 
The stage involved reviewing of previous literature on the area, conducting of research 
about the general geography and previous work in the study area. An important exercise 
carried during the desk study was the reproduction of the topographical base map of the area 
by enlargement. The enlargement of 1:25000 was done from the original topographic maps 
with sheet numbers 0500A1 and map scale of 1: 50000 obtained from the Ghana Survey 
Department.  
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3.2 Tools and Equipment 
Standard geological field tools were used during the mapping exercise. Among these tools 
was a standard geological compass and Global positioning system (GPS) which was mainly 
used for finding direction, self-location and self-orientations and the measurement of 
attitudes of structures. Other tools used include geological hammer for breaking and 
chipping rocks, tape measure for taking measurement of outcrops, digital camera for taking 
pictures of outcrops, cutlass for creating paths in inaccessible areas and for clearing weeds 
around hidden outcrops, and a geological field gear including a pair of safety boots for 
ensuring safety in the field.  
3.3 Field Methods and Sampling 
Several field methods were used during the entire period of mapping. Field methods such as 
a reconnaissance survey, setting up a base camp, choosing of relevant reference points, self-
location, self-orientation, self-positioning, Global positioning system (GPS) -and-compass 
traversing, outcrop description and sampling were all carried out to facilitate a smooth and 
successful mapping of the study area. 
Fresh, representative and orientated rock samples were taken from the field and to the 
laboratory for subsequent analyses (i.e., petrographic and geochemical analyses). The 
samples from the field were further sorted, grouped and carefully selected for thin section 
preparations 
 
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3.3.1 Reconnaissance Survey 
On the field, a reconnaissance survey was conducted early in the study area especially in 
places close to the base camp in order to become fairly acquainted with the area. This 
exercise was done in two days. During the exercise I walked in and around the community 
in which the base camp was located observing the general topography, climate and 
vegetation. I also familiarized myself with local people to inform them about my mission in 
their locality; by this, vital information was obtained about the accessibility and cultural 
practices of the area. 
3.3.2 Mapping Procedures 
The main mapping procedures constitute the major part of the whole field work. The 
mapping procedures used included the Global positioning system (GPS)-and-Compass 
traversing technique, outcrop description techniques and plotting of field data. However, 
since the geological compass was used in almost all the mapping procedures it had to be first 
corrected to produce accurate. 
3.3.2.1 Compass Correction 
The geological compass used for the survey was corrected using the required compass 
correction information provided on the topographic base map of the area. This information 
included the magnetic declination data which includes the value of the deviation of the 
magnetic north from the grid north and the value of the annual declination as well as the date 
of production of the map. With this information the magnetic declination of the area with 
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respect to the base map was calculated and the compass was thus corrected accordingly by 
shifting the adjusting screw. The geological compass was adjusted to align the magnetic 
north with the grid north to ensure accuracy. 
3.3.2.2 Traversing 
The main survey method used on the field was the Global positioning system (GPS) and 
Compass traversing method. With the help of the base map, GPS and corrected geological 
compass I established suitable reference points in each of the divisions. In each of the 
divisions systematic traversing was done to locate and identify rock outcrops. With the help 
of the base map, the GPS and compass as the primary guide, I traversed the study area using 
all the available accessible routes. However, in the places where there were no accessible 
routes to potential outcrop locations, paths were created with the help of the cutlasses. 
Traversing was mostly done along footpaths, road cuts and river channels, and often in 
closed loops across strike from a known location towards a prominent feature along a 
particular bearing. The geological compass was used to find the bearing any time there was 
a change in direction during the traversing. 
3.3.2.3 Outcrop description techniques. 
At every outcrop site or station, standard petrographic field description techniques were used 
to describe and classify the rocks. The dimensions of mesoscopic outcrops were taken with 
the help of the tape measure. Attitudes of structures including the strike and dip of foliations 
and joints were measured with compass, whilst the hammer was used to chip of oriented 
samples for further petrographical analyses. These samples were labeled and the measured 
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attitude values written on them where necessary. Large enough representative samples were 
taken of the rocks to cover mineralogical and structural features of the outcrops; mostly they 
were about 5 Kg. All measurements were recorded in the field notebook at the outcrop 
station before proceeding to new outcrops. This was done to ensure the authenticity and 
reliability of the data taken from the field. Sketches of spectacular features of the outcrops 
were also made in the field notebooks, whilst the cameras were used to take photographs of 
other vital features of the outcrops. 
3.3.2.4 Data Plotting 
After every field session, the data collected were adequately reviewed, analyzed and plotted 
on the appropriate overlays. The field traversing data were plotted on the traverse map, the 
outcrop data on the outcrop map and structural data were plotted on the structural map. The 
labels and the characteristics of the samples obtained were also compared with what were 
recorded in the notebooks to ensure accuracy and consistency. 
3.4 Post Field Procedures 
Post field procedures involved preparations of rocks samples for further petrological and 
microstructural analysis as well as the analysis of field data and further plotting and 
geochemical analysis on the sample taken. 
 
 
 
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3.4.1 Thin section Preparation 
Thin sections were prepared from thirty-Five (35) selected rock samples at the Earth Science 
Department, University of Ghana. The selected samples were first cut into thin slices of 
about 3 mm thick. Samples that were not hard enough to provide the required slices were 
impregnated with cohesive cement to harden them before they were cut. Cutting was mostly 
done across mineral grains in order to obtain enough information from the thin sections. For 
each rock sample at least two slices were cut, in order to obtain two thin sections from each 
sample. Next, the surfaces of the rock slices were polished with abrasives and mounted on 
thin wet glass surfaces. One side of the glass slides was also polished to obtain a smooth 
surface before the mounting was carried out. The surface was cleaned and made perfectly 
flat and free of grooves and pits. A thin layer of Canada balsam was spread over the surface 
of the rock slice and the glass slide and evaporated to a preferred hardness. Rock slice and 
glass slide were removed quickly and the balsam coated surfaces were put in contact. The 
slice and slide were pressed together to remove bubbles and allow the mount to cool. 
Thickness of the thin section made was 0.03 mm. 
The rock texture in the prepared thin sections was studied under the petrographic 
microscope. Photomicrographs were taken of the minerals and micro-structures in the thin 
section. Through thin section analyses invisible minerals in hand specimen were identified 
and quantitative measurements of percentage mineral content were made usingpoint 
counting. Rocks names were assigned to the rock samples based on hand specimen and 
petrographic results. 
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3.4.2 Geochemical Analysis 
Based on the results from the petrographic study, twenty-five (25) fresh, relatively unaltered 
and representative samples were selected and sent to the ALS laboratory, Vancouver-
Canada for their whole rock major and trace elements (including REEs) analyses. Major and 
trace elements analyses were carried out by inductively coupled plasma atomic emission 
spectrometry (ICP-AES) and multi elements fusion inductively coupled plasma mass 
spectrometry (ICP-MS) respectively. Loss of ignition was determined at 1000 0C. For the 
major elements analyses, a mixture of about 0.200g of prepared sample and lithium 
metaborate was fused in a furnace at 10250C. The resulted melt was then cooled and 
dissolved in an acid mixture. The acid mixture contained nitric, hydrochloric and 
hydrofluoric acids. The solution was then analysed by ICP-AES for the major element 
composition. The trace element analyses were performed observing the protocols as done for 
the major element analyses. However, here the prepared sample weighed 0.100 g and the 
analysis was done by the ICP-MS. The base metals were analysed using ICP-AES. A 0.25 g 
prepared sample was digested with perchloric, nitric, hydrochloric and hydrofluoric acids. 
Dilute hydrochloric acid was then added to the residue and the solution was analysed by the 
ICP-AES. Results obtained were corrected for spectral interferences (ALS laboratory). 
Precision is better than 2%. 
 
 
 
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3.4.3 Map Digitization, Stereographic projections and Cross Section Drawing. 
The various maps obtained from the field were digitized with computer software (e.g. Map 
Info and Arc GIS). This was done in order to help in the construction of a composite 
geological map of the area. With the complete geological map of the area a cross-section 
was drawn through all the lithologies showing their distribution and relationships with the 
various geological structures. Stereographic plots were made using the Stereopro to 
determine the general behavior of structures in the area. 
 
 
 
 
 
 
 
 
 
 
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CHAPTER FOUR 
4.0 RESULTS 
4.1 Field Relation and Petrography 
Outcrops within the study area were mainly mafic garnet gneisses with NE strike and gentle 
dip to the east. The rocks appear to have experienced intense deformation and shearing.  
The mafic garnet gneisses (Fig 4.1) are divided into pyroxene garnet gneisses and garnet 
hornblende gneisses based on their mineral compositions. The pyroxene garnet gneisses are 
the most dominant. The pyroxene garnet gneisses outcrop mainly in the Naglayo Hill and 
Shai forest reserve area.  The hornblende garnet gneisses outcrop mainly at the Mawum and 
Eastern quarries at the Mampong Hill.  
These mafic garnet gneisses are associated with pyroxenite, pyroxene bearing amphibolites 
and a fault rock. The pyroxenite occurs as older materials thrusted within mafic garnet 
gneisses. They are observed throughout the hills in the study area. Some form boudins 
within the gneisses. A fault rock was found within a fault at the PW Quarries in the Naglayo 
hill. The pyroxene bearing amphibolite was observed at the Eastern Quarries at the 
Mampong Hill. They are tectonically welded within the mafic garnet gneisses.  
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Figure 4.1: Outcrop map of the study Area. 
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4.1.1 Mafic Garnet Gneisses 
The mafic garnet gneisses generally occur as inselbergs and scattered outcrops and 
represented the most dominant lithology across the study area. These inselbergs are the 
Mampong, Naglayo and the Shai Hills which are trending North-South. Several quarries in 
the study area provide access to fresh outcrops for detailed studies and sampling.  
These mafic garnet gneisses are medium to coarse grained and are generally greyish, and 
sometimes reddish, with garnetiferous facies or true garnetites. The mafic gneisses exhibit 
gneissic banding. The rocks have alternating white and dark bands (Fig 4.2a). The garnet 
crystals are millimetric to centimetric and probably of several generations (Fig 4.2c). Their 
great abundance is the outstanding characteristic of these gneisses. The predominant 
minerals in these gneisses are amphiboles, quartz, plagioclase feldspar, pyroxenes, garnet 
and some opaque minerals. They contain shreds of pyroxenites and pyroxenite boudins. The 
mafic layers present in the rocks are rich in pyroxene and hornblende whilst the felsic layers 
are rich in feldspar and quartz. At some places, some garnet bands are observed in the mafic 
garnet gneisses.  
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Figure 4.2: Field photographs showing (a) subhorizontal layers of mafic and felsic bands in 
mafic garnet gneiss; at Eastern Quarries, Mampong Hill (b) Compositional felsic bands in 
the mafic garnet gneiss at Mampong Hill (c) Porphyroblastic garnet of different sizes at PW 
Quarries, Naglayo Hills (d) Alternating bands of mafic, felsic and garnet bands in the mafic 
garnet gneiss at the West Coast Quarries, Naglayo Hills.   
  
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The rocks generally trend in the northeast-southwest direction. The mafic garnet gneisses are 
more strongly deformed, sheared and recrystallized. There are prominent discontinuous 
layerings which consist of garnet rich and hornblende-rich zones in some locality which give 
the rock a streaky appearance and are interpreted to be shear induced. The alternating 
parallel to sub-parallel dark and white (quartzo-feldspathic) layers or bands centimeters to 
meter thick are prominent in the area and well exposed as a results of the quarrying activities 
in the area. The white silica rich layers are made up of feldspars most predominantly 
plagioclase, quartz and hornblendes form compositional banding and together with and the 
mafic layers define the foliations S1 planes.  
The hornblende porphyroclasts within the compositional bands are well-developed and 
stretched into sigma and delta-type tails (Fig 4.3a). Some of the compositional bands are 
folded into ptygmatic folds (Fig 4.3c). These folded compositional bands occur in all sizes 
and orientations to the tectonic layering; some prominent once which attain a thickness of 
0.5 m and together with the thin bands, which are only a few millimetres thick, are estimated 
to make up to 10% of the rock by volume (Burke, 1959). Pressure shadows are mostly 
observed around large garnet porphyroblasts (Fig 4.3d). Some of these garnet porphyroblasts 
are wrapped around by the S1 external foliation. A halo of feldspar is mostly observed 
around large garnet porphyroblasts. In some localities, some of the pyroxenite layers are 
well sheared and stretched in the mafic garnet gneisses as a result of shearing (Fig 4.4a).   
The Hornblende porphyroclasts with well-developed sigma and delta type tails within the 
thick bands, together with asymmetrical pressure shadows around garnet porphyroblasts and 
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sheared pyroxenite layer form spectacular sets of kinematic indicators. There are several 
brittle vertical to subvertical normal faults and fractures (Fig 4.4b). A fault rock comprising 
a 0.5–3.0 m thick zone of dark green, chlorite-rich breccia is observed at the PW Quarries 
(Fig 4.4c).  
 
 
 
 
 
 
 
 
 
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Figure 4.3: Field Photographs showing (a) compositional bands with stretched hornblendes 
(b) Compositional bands folded into ptygmatic fold (c) Ptygmatic folds (d) Pressure Shadow 
formed at the edges of porphyroblastic garnet minerals. 
 
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Figure 4.4: Field photographs showing (a) stretched pyroxenite within the mafic garnet 
gneisses at the Shai Hill Game Reserve (b) brittle fault at the PW Quarries; Naglayo Hill (c) 
brittle fault zone observed at PW Quarries at Naglayo Hill (d) Joints found at the Shai Hill 
game reserve. 
  
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Under the microscope, the mafic garnet gneisses are medium to coarse grained and show 
recrystallized texture. The minerals observed are mainly; garnet, pyroxene, plagioclase, 
hornblende, quartz, ilmenite and rutile. In these rocks, the porphyroblasts are represented by 
millimetric sections of garnet, and clinopyroxene clearly oriented in granoblastic to 
granonematoblastic groundmass of plagioclase and quartz (Fig 4.5a and b). The main facies 
of the mafic garnet gneisses include lenticular clinopyroxene porphyroblasts, garnet 
porphyroblast and plagioclase.  
Clinopyroxene 
The clinopyroxene minerals display deformed cleavage microfractures, undulose extinction 
and large rims or overgrowths of green hornblende (Fig 4.6). The clinopyroxenes under 
plane polarized light is pale green in colour, has low relief and shows subtle greenish 
pleochroism. The minerals are highly fractured and mostly appear broken. Some 
clinopyroxenes have inclusions of hornblende and ilmenite. Furthermore, some of the 
clinopyroxenes have rims of hornblende and some with rims of chlorite (Fig 4.6a). 
Inclusions of clinopyroxenes are observed in poikiloblastic euhedral garnet crystals (Fig 
4.8b). A corona texture of clinopyroxene surrounds porphyroblastic garnet (Fig 4.8f). The 
clinopyroxene porphyroblasts are been wrapped by S1 foliation. 
Garnet 
Garnets are abundant, both as tiny crystals scattered throughout and as larger 
porphyroblasts. The garnet porphyroblast vary in shape and size. Garnet grains have 
brownish absorption colors but goes dark with no extinction under crossed polars. The 
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shapes of the garnet are anhedral to euhedral. The garnet porphyroblasts mainly display 
sieve structure or poikiloblastic texture owing to the inclusions of hornblende, 
clinopyroxenes, ilmenite and rutile. This feature is very well displayed by the garnet 
porphyroblasts in the thin section (Fig 4.5d) and (Fig 4.8b).  
Some of the garnet are highly deformed and are related to multiple posterior tectonic phases 
in the mafic garnet gneisses. According to the shape of the garnet, their relationship with the 
S1 foliations and their inclusions, at least two generations of garnet can be defined. The first 
garnet is the pre kinematic garnet, followed by and syn to post kinematic garnets. The pre 
kinematic garnet type are most deformed and surrounded by the S1 foliation (Fig 4.8a). 
Some have inclusions of hornblende and ilmenite. They are highly deformed, broken with 
anhedral shape. One of these garnets displays a corona texture around it. The syn kinematic 
garnet is the most abundant garnet in the mafic garnet gneisses. The syn kinematic garnets 
have inclusion of clinopyroxenes, hornblende, rutile and plagioclase (Fig 4.8b). The syn 
kinematic garnets are relatively less deformed and are most abundant in the mafic garnet 
gneisses and have euhedral to subhedral shapes. Some garnet megacrysts (diameter of about 
>1cm) contain rutile ex-solution rods (Fig 4.8c). Rutile ex-solution rods are lamellar or rod-
like intergrowths of rutile arranged in a somewhat triangular pattern in garnet.  
  
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Plagioclase 
Plagioclase and quartz constitute the leucocratic and granoblastic groundmass in these mafic 
garnet gneisses. The plagioclase is andesine plagioclase. Some of the plagioclase grains are 
more or less porphyroblast. They are deformed with bent twinning. They display 
polysynthetic flexural twins and spectacularly abnormal extinction (Fig 4.9b). Most of the 
plagioclases are associated with quartz materializing the S1 foliations. The plagioclases are 
colourless, show low relief and exhibit polysynthetic twinning. 
Quartz 
The quartz crystals are colorless under plane polarized light and lack cleavages. They show 
sutured boundaries, exhibit undulose extinction and are associated with plagioclase forming 
granoblastic textures. The quartz crystals often define the gneissic banding (S1). 
Hornblende 
Hornblende under plane polarized light is distinctly coloured and display strong yellowish to 
greenish pleochroism. However, it assumes a dark greenish brown color. Some hornblende 
shows cleavage in two directions typical of amphibole cleavage of 1200/60o intersection.  
Hornblende crystals observed are mostly anhedral. Inclusions of the hornblendes occur in 
the garnet porphyroblast (Fig 4.5c). Some of the hornblende form rims around pyroxenes 
and garnet (Fig 4.6). Some dark reddish brown hornblende is well represented in S1 foliation 
and defines the L1 mineral lineations (Fig 4.9a).  
 
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Chlorite and Epidote 
Fibrous pale green chlorites also occur in the mafic garnet gneisses around some 
clinopyroxenes and garnets porphyroblast. Also some plagioclase minerals seem to alter into 
epidote which exhibit faint yellow taint under plane polars. 
Accessory Minerals 
Ilmenite occurs quite frequently as xenomorphic clusters or opaque inclusions garnet 
porphyroblasts. Rutiles also occur as exsolution rods in the garnet porphroblast (Fig 4.8c).  
Ilmenite and rutile occur as accessory minerals in these rocks.  
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Figure 4.5: (a) Photomicrograph showing garnet minerals. Plane polarized light (Ppl) (b) 
Photomicrograph showing garnet and clinopyroxene porphyroblast (Ppl) (c) Garnet 
porhyroblast with hornblende inclusion (Ppl) (d) Poikiloblastic texture of garnet with 
inclusions of quartz and opaque minerals (Ppl). Garnet (Grt), Plagioclase (Pl), Microcline 
(Mc), Pyroxene (Px), Clinopyroxene (Cpx), Orthopyroxene (Opx), Hornblende (Hbl), 
Quartz (Qtz), Chlorite (Chl), Opaque mineral (Op). 
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Figure 4.6: (a) Photomicrograph showing Clinopyroxene porphyroblast with rims of chlorite 
with the S1 foliation around it. (Ppl). (b) Photomicrograph showing Clinopyroxene 
porphyroblast with rims of chlorite. (Xpl).  (c) Hornblende rims around broken 
Clinopyroxene (Ppl) (d) Hornblende rims around broken Clinopyroxene (Xpl). Same 
Abbreviations are used. 
 
 
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Figure 4.7: (a) and (b) Photomicrographs are showing porphyroblastic Clinopyroxene with 
rims of hornblende intracrystalline fractures. (Ppl) (c) and (e) Broken and cracked 
Clinopyroxene porphyroblast (Ppl) (d) and (f) Broken Clinopyroxene porphyroblast. (Xpl). 
Same Abbreviations are used. 
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Figure 4.8: (a) Photomicrogragh showing deformed pre kinematic garnet porphyroblast with 
rims of hornblende and S1 foliation wrapping around it. (Ppl). (b) Syn kinematic garnet with 
inclusion of Clinopyroxenes. (Xpl). (c) Garnet with rutile exsolution (Xpl). (d) Syn 
kinematic garnets (Ppl). (e) Garnet porphroblast with inclusions of quartz and plagioclase 
(Xpl).  (f) Corona texture of Clinopyroxenes and quartz around garnet porphyroblast (Ppl). 
Same Abbreviations are used. 
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Figure 4.9: (a) Photomicrograph showing L1 mineral lineation in the S1 foliation. (b) 
Photomicrograph showing deformed polysynthetic twining in plagioclase and deformed 
quartz. (Ppl). Same Abbreviations are used. 
 
The mafic garnet gneisses are distinguished into pyroxene garnet gneisses and garnet 
hornblende gneisses based on the modal composition of the studied samples.  
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Figure 4.10: Photomicrograph showing garnet hornblende gneiss. (Xpl) Same Abbreviations 
are used. 
Table 1:  Average modal composition (vol %) of minerals in the garnet hornblende gneiss. 
Mineral Modal composition 
Hornblende 30 
Plagioclase 25 
Garnet 15 
Clinopyroxene 13 
Quartz 7 
Chlorite 4 
Epidote 3 
Accessory mineral (Ilmenite and rutile) 5 
 
 
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The pyroxene garnet gneisses are most abundant in the study area. They have the same 
minerals composition but different modal composition as the hornblende garnet gneisses.  
 
Figure 4.11: Photomicrographs of pyroxene garnet gneiss. (Xpl). Same Abbreviations are 
used. 
 
 
 
 
 
  
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Table 2: Average modal composition (vol%) of minerals in the pyroxene garnet gneiss. 
Mineral Modal composition 
Hornblende 10 
Plagioclase  22 
Garnet 30 
Clinopyroxene 20 
Quartz 6 
Chlorite 4 
Epidote 3 
Accessory minerals (Ilmenite and rutile) 5 
 
4.1.2 Pyroxene Bearing Amphibolite 
The pyroxene bearing amphibolites are found at the base of the mafic garnet gneisses at 
Eastern Quarries at the Mampong Hill. These rocks are tectonically welded with the mafic 
garnet gneisses. They are strongly deformed and sheared together with mafic garnet 
gneisses. They look massive at the base of the mafic garnet gneisses and gradually transpose 
into the foliations plane of the mafic garnet gneisses. The contact between the two rocks is 
gradational. The amphibolite is a dark grey coloured, medium-grained rock, essentially 
composed of dark greenish-black hornblende and feldspar with some garnet. 
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                 Figure 4.12: Field photograph of pyroxene bearing amphibolite. 
 
                        
 
Figure 4.13: Field photograph of amphibolite tectonically welded into host mafic garnet 
gneiss at Eastern quarries, Mampong Hill. (b) Gradational contact between the amphibolite 
and the mafic garnet garnet gneiss. 
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Under the petrographic microscope, minerals observed are amphiboles, pyroxenes, garnet, 
plagioclase and quartz. The amphiboles are distinctively coloured. They exhibit deep green 
absorption colors with some exhibiting a typical 60o/120o amphibole cleavage. They show 
deep green to light green pleochroism. The amphiboles are anhedral in shape. The 
plagioclases are colorless under plane polar. Some grains show multiple twinning which 
may indicate deformation during recrystallization. The pyroxenes exhibit yellowish green 
absorptions colors which undergo pleochroism to green upon rotation of the stage. These 
pyroxenes are clinopyroxenes. They are anhedral to subhedral in shape. A few garnet grains 
are observed in the thin section. The garnets have subhedral shape. The garnet crystal 
display sieve structure or poikiloblastic texture with inclusions of hornblende. Few quartz 
minerals are also observed. The grains of quartz are colorless and shows sutured boundaries. 
They undergo undulose extinction under cross polars when the stage is rotated. The mineral 
assemblage of the pyroxene bearing amphibolite indicates amphibolite facies at higher 
temperature and pressure. 
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Figure 4.14: Photomicrographs of pyroxene bearing amphibolite. (Ppl). Same Abbreviations 
are used. 
  
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Table 3: Average modal composition (vol %) of minerals in pyroxene bearing Amphibolite. 
Mineral Modal composition 
Amphibole (hornblende) 55 
Pyroxene 20 
Plagioclase 15 
Garnet   5 
Quartz   3 
Opaque mineral 2 
 
4.1.3 Pyroxenite 
The pyroxenites associated with mafic garnet gneisses are seen as older materials thrusted 
into gneisses. Some form boudins in the gneisses. The pyroxenites rocks are thrusted into 
the mafic garnet gneisses. They consist of dark pyroxene minerals. Some form boudins in 
the mafic garnet gneisses at some localities. These pyroxenites are also tectonically 
deformed together with the mafic garnet gneisses.  
 
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Figure 4.15: Field photograph showing pyroxenite thrusted into the mafic garnet gneiss at 
Eastern Quarries. Mampong Hill. 
 
 
Figure 4.16: Field photograph showing pyroxenites boudins in the mafic garnet gneiss at 
PW Quarries. Naglayo hill. 
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The pyroxenites under the microscope display granoblastic and quite equant textures. They 
are made up of variable proportions of orthopyroxenes and clinopyroxenes. These minerals 
display deformed cleavages. They are highly fractured with intracrystalline fractures. The 
clinopyroxenes are pale green and shows pleochroism. The exhibit pale green to deep green 
upon rotation of the stage under plane polars. The orthopyroxenes exhibit pale green to pink 
pleochrosim. 
 
Figure 4.17: (a) and (c) Photomicrographs showing Clinopyroxene (Cpx) and 
Orthopyroxene (Opx) in the pyroxenite. (Ppl). (b) and (d) Photomicrographs showing 
Clinopyroxene (Cpx) and Orthopyroxene (Opx) in the pyroxenite. (Xpl). Same 
Abbreviations are used. 
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Table 4: Average modal composition (vol%) of minerals in pyroxenite. 
Mineral Modal composition 
Pyroxene 95 
Opaque minerals 5 
 
4.1.4 Fault Breccia 
The brecciated rock is found within the mafic garnet gneisses at the PW Quarries, Naglayo 
Hill in the study area. They are found in a brittle fault zone in the area (Fig. 4.4c). The fault 
rocks comprise a 0.5–3.0 m thick zone of dark green, chlorite-rich breccia is observed at the 
PW Quarries. The rocks represent a crushed garnet mafic gneisses during a brittle 
deformational event in the area. Clast of quartz minerals of variable shapes and sizes are 
observed within the matrix in the rock. The fault zone had strike of N3100W with dip 
400SW. 
Under the microscope, the rock exhibited clasts of quartz within broken fine grains of 
pyroxenes and chlorite matrix. These quartz clasts are anhedral to subhedral in shape. They 
exhibited undulose extinction. The matrix is made up of fine and crushed grains of 
pyroxenes with some chlorites. The rock exhibits a mortar texture where porphyroclast of 
quartz are surrounded by crushed matrix. 
 
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Figure 4.18: Photomicrograph of the Fault Breccia. (Ppl). Same Abbreviations are used. 
 
Table 5: Average modal composition (vol%) of minerals in fault breccia. 
Mineral                                                                                    Modal composition 
Quartz                                                                                               20 
Matrix                                                                                               80 
 
  
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4.2 Deformations, Structures and Kinematics 
The exposure of outcrops in the study area provided detailed information on the structural 
setting on the study area. The structures have been statistically analyzed and their probable 
causes and relations have been established. The area has undergone several regimes of 
tectonic and metamorphic events; thus, the rocks show numerous deformational structures. 
The mafic garnet gneisses together with the related rocks in the study area are tectonically 
sheared. They have undergone several regimes of metamorphic and deformational events 
that have caused the rocks to undergo significant textural and structural changes resulting in 
the development of tectonic structures with kinematic indicators. The structures include 
bands and foliations, folds, faults, boudins, mineral lineations.  
4.2.1 Gneissic Bands and Foliations. 
The foliations in the mafic garnet gneiss are well represented by the gneissic bands found in 
the rocks. The alternating gneissic bands in the rock are well exposed as a result of the 
quarries activities in the study. They were formed probably during the D1 deformational 
event and produced S1 foliations fabric. These structures occur as repetitive layering or 
alternating dark and light minerals caused by shearing forces or differential pressure acting 
on a rock mass. The gneissic bands generally formed by the reorganisation of the original 
rock's chemical constituents into layers within which particularly minerals are concentrated. 
Foliations are more pronounced in the mafic garnet gneisses. The felsic compositional band 
is composed of plagioclase feldspar, quartz and hornblende. The hornblende mineral defines 
L1 mineral lineation during the D1 deformation. At certain place garnet bands are also 
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observed which are sub parallel to the alternating light and dark bands. The foliations have a 
NE–SW (average at N20E) foliation trend with gentle dips of average of 15o toward SE. 
Few exceptions show NNW strike (Table 6). The general north easterly strike indicates NW 
to SE simple or non-coaxial shearing during the D1 deformation. The S1 foliations are well 
exposed in the mafic garnet gneisses. 
 
Figure 4.19: Field photographs showing gneissic banding and foliations in the mafic garnet 
gneisses. Shai hills. 
 
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Table 6:  Structural attitude (degree) of foliation planes. 
Strike  Dip  Dip Direction  
                         20                         15                        124 
15 20 120 
28 15 100 
20 20 110 
16 20 130 
40 15 126 
340 15 30 
350 20 40 
   
 
From the poles plots (Fig 4.20a) it shows that the foliation in the mafic garnet gneisses have 
predominantly southeasterly dip direction with gentle to moderate dips. The plane to pole 
(Fig 4.20b) indicates that the rock mainly dips to southeastern direction with average dip 
been moderate thus about 15o. The contoured poles density plot (Fig 4.21) indicates majority 
of the poles are concentrated closer to the center, which show that the foliations in the area 
are of gentle dip. 
 
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Figure 4.20: (a) Poles plot of foliations in mafic garnet gneisses (b) Poles plot with plane to 
poles plot in the mafic garnet gneiss. 
 
Figure 4.21: Contoured poles-density plot of foliations in mafic garnet gneisses. 
 
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4.2.2 Mineral lineation. 
The mineral lineations are well represented by hornblende within the compositional felsic 
bands in the mafic garnet gneisses. The L1 hornblende minerals are developed into sigma (σ) 
and delta (δ) giving indications of simple shearing probably during during the D1 
deformation stage. This structure also gives shear sense indications during the D1 
deformation of the mafic gneisses. These hornblende minerals are stretched within the 
quartzofeldspathic compositional bands. The stretching lineation observed on the foliation 
planes shows a NE–SW trend direction with gentle plunge (average of about 200) towards 
the NE or SW (Table 7) 
 
Figure 4.22: Stretched hornblende within compositional banding in the mafic garnet gneiss. 
 
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      Table 7: Structural attitude (degree) of mineral lineation. 
Trend                          Plunge  
                         215                           20 
200 15 
208 30 
200 20 
206 24 
20 16 
34 18 
212 15 
 
The poles plots (Fig 4.23) indicate NE–SW trends of the L1 mineral lineation which gently 
plunges. It gently plunges to the SW with some plunging gently to NE. 
 
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Figure 4.23: Shows Poles plot of lineations in mafic garnet gneisses. 
  
Shear sense indicators during the D1 are mainly dextral, represented by:  
 Sheared pyroxenite boudin rock within the mafic garnet gneiss (Fig 4.24) 
  Stretched hornblende mineral lineations into sigma and delta structures (Fig 
4.22). 
 Clockwise rotation of garnets minerals. (Fig 4.25) 
 
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Figure 4.24: Photograph showing dextral sense of shearing by sheared pyroxenite boudin in 
the mafic garnet gneiss during D1 deformation at Shai Forest reserve. 
 
 
Figure 4.25: Photograph showing dextral sense by clockwise rotation of garnet in the mafic 
garnet gneiss during D1 deformation at PW Quarries, Naglayo Hill. 
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4.2.3 Ptygmatic Folds 
In the mafic garnet gneisses, the compositional banding is folded into ptygmatic folds.  
These folds probably might have been formed during D2 deformational phase in the mafic 
garnet gneisses. They postdate D1 deformation. These folds are observable almost 
throughout mafic garnet gneisses. These D2 ptygmatic folds cross cut foliation (Fig 4.19c 
and Fig 4.3c). Upon axial measurements of these folds, it was observed that they are of 
different orientations. Some have NE/SW trend and others with NW/SE trends with average 
of 25o plunge (Table 8). They crosscut the S1 foliations which show that they are younger 
than the foliations or postdate the S1 foliations. 
 
Table 8: Structural attitude (degree) of Ptygmatic fold axis. 
                             Trend                               Plunge  
                                200                                  30 
210 20 
214 25 
212 22 
200 30 
190 24 
124 40 
120 25 
 
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From the stereographic poles plot below, these folds are of different direction. They trend 
SW/NE and SE/NW with average plunge of 150 SW and SE respectively.  
 
 
Figure 4.26: Poles plot of fold axis of ptymatic folds. 
 
 
 
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4.2.4 Faults 
The subvertical faults found through the mafic garnet gneisses represent the D3 deformations 
in the area. These brittle faults cut through the S1 foliation and the D2 folding. This indicates 
that the faults are younger event in the mafic garnet gneisses. These faults seen mainly in the 
area are normal faulting where the hanging wall have move down relatively to the footwall 
as a result of extension during decompression. These normal dip slip faults have moderate 
dips with two different direction of dip. These faults dip SW and strike NW/SE with some 
having a NW dip direction and NE/SW strike (Table 9). These faults have maximum 
displacement of about 30 cm. 
 
Figure 4.27: Field photograph showing normal faults in the mafic garnet gneiss at PW 
Quarries, Mampong Hill. 
 
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Table 9: Structural attitude (degree) of faults planes in the area. 
Strike  Dip  Dip Direction  
                            312                              40                             215 
302 42 200 
350 40 208 
310 34 240 
322 44 232 
70 38 300 
72 40 312 
50 32 320 
 
 
 
 
 
 
 
 
 
 
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From the stereographic pole plots, it can inferred that there are indicates two dip directions 
of the fault planes with most dipping SW and having moderate dips average of about 350 to 
400 (Fig 4.28). 
 
Figure 4.28: Poles plot of faults in the mafic garnet gneisses. 
 
 
 
 
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These subvertical faults postdate the ductile deformations and cuts through the main 
foliations, the ptygmatic folds. The above structures represent a ductile–brittle continuum of 
extensional deformation of the mafic garnet gneiss during decreasing temperatures and 
pressures during exhumation. The thrusted rigid older materials (pyroxenite) within the 
mafic garnet gneiss are fractured and exhibit dextral displaced grains but have overall 
sinistral sense of shearing during D3 deformation (Fig 4.29). 
 
Figure 4.29: Photograph showing overall sinistral sense of shear during the D3     
deformations. 
 
 
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4.2.5 Joints 
Joints are fractures or cracks in a rock body which show no appreciable level of openings. 
These joints also postdate the foliations and ptygamtic folds in the rocks. They are usually 
found in the mafic gneisses (Fig 4.4d). Joints form at shallow depths in the crust where 
rocks break in a brittle way and are pulled slightly apart by tensional stresses during 
extension.  
4.2.6 Boudins 
Pyroxenite boudins were found within the mafic garnet gneisses. These are older pyroxenite 
materials that were thrusted in to the mafic garnet gneisses during the main D1 deformation. 
The boudins form as a response to layer parallel extension or layer perpendicular flattening 
of the stiff pyroxenite materials. They stretch as a result of ductile contrast of less competent 
participating layers or rocks (Fig 4.16). 
 
 
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Figure 4.30: Structural map of the study area. 
 
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4.3 Major and Trace Element Geochemistry 
The whole rocks major and trace element compositions of 23 rock samples from the study 
are presented in Table 10, Table 11 and Table 12. Two (2) samples of pyroxene bearing 
amphibolite, one (1) fault breccia, one (1) pyroxenite rock sample and nineteen (19) mafic 
garnet gneisses rock samples of which seven (7) samples are garnet hornblende gneisses and 
twelve (12) are pyroxene garnet gneiss rock samples.  
4.3.1 Major Element Geochemistry 
4.3.1.1 Pyroxene Garnet Gneisses 
The concentration of SiO2 ranges from 49.5 to 57.2 wt.%. The rocks have high Al2O3 
content of 18.35 to 23.8 wt%. TiO2 of 0.56 to 1.09 wt.%.  Fe2O3t from 2.06 to 10.25 wt.%, 
CaO content of 7.44 to 9.06 wt.%, MgO of 1.18 to 6.65 wt.%, Na2O content of 4.1 to 6.68 
wt.%, K2O of 0.26 to 0.56 wt%, CrO3 of 0.01 to 0.02wt%, MnO from 0.01 to 0.19, P2O5 of 
0.06 to 0.26 wt.%, SrO from 0.08-0.18 wt.%, BaO from 0.01 – 0.03 wt.% (Table 10). The 
Mg# ranges from 50.63 to 60.30%. Samples with higher Mg# have relatively lower Al2O3 
content. Some of the samples have TiO2 >1.0 wt%. These basaltic rocks are depleted in K2O 
(K2O of 0.26-0.56 wt.%,), but relatively higher Na2O content of 4.1-6.68 wt.%. These rocks 
have high composition of aluminium (high-alumina), Low-K tholeiitic basalt (Table 10). 
 
 
 
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4.3.1.2 Garnet Hornblende Gneisses. 
The garnet hornblende gneisses have similar major elements concentrations just as the 
pyroxene garnet gneisses (Table 11). The SiO2 ranges from 47.7 to 51.1wt%. These rocks 
just as the pyroxene garnet gneisses are characterized by high alumina content from 17.9 to 
20.3 wt%. Fe2O3 from 8.64 to 11.9 wt.%, CaO of 8.11 to 9.65 wt.%, MgO ranges from 4.67 
to 6.75wt.%, Na2O 4.02 to 4.97 wt.%, low K2O from 0.33 to 0.48 wt.%, Cr2O3 0.01 to 0.02 
wt.%, TiO2 0.89 to 1.76 wt.%, MnO 0.08 to 0.15 wt.%, P2O5 of 0.15wt%, SrO from 0.07 to 
0.1wt%, BaO 0.01 to 0.02. The Mg# ranges from 50.72 to 57.99 (Table 11). 
4.3.1.3 Fault Breccia. 
The fault breccia which is found in a fault zone in the pyroxene bearing gneisses at PW 
Quarries also has the similar characteristics as the host rock. One sample of the fault rock 
was analysed. The result (Table 12) shows that the breccia is characterized by SiO2 of 50.1 
wt.%, high alumina content of 17.35 wt.%, Fe2O3 of 6.99 wt.%, high CaO content of 19.05 
wt.%, MgO content of 2.88 wt.%, Na2O of content of 0.11wt.%, low K2O content of 0.04 
wt.%, Cr2O3 of 0.01wt.%, TiO2 of 0.58 wt.%, MnO of 0.09 wt.%, P2O5 of 0.08wt.%, SrO of 
0.15wt.%, BaO of 0.01wt%, The Mg# of 49.94. 
4.3.1.4  Pyroxene bearing Amphibolite. 
Two samples were analysed. These rocks are characterized by low silica content (Table 12). 
The SiO2 contents are 41 and 40.6 wt.% which are representative of ultrabasic rock. They 
were 14.1 and 14.35 wt.% alumina content relative lower compared to the mafic garnet 
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gneisses. They have higher Fe2O3t content with 14.65 and 14.8 wt.% concentration with also 
high content of CaO content of 11.4 and 11.35wt.%. The MgO content is high 11.2 wt.% 
and 10.9 wt.%. Relatively low Na2O content of 2.76 and 2.73 wt.%. K2O of 0.59 and 0.58 
wt.%, Cr2O3 is 0.03 and 0.02 wt.%, TiO2 is 2.62 and 2.91 wt.%, MnO of 0.13 and 0.13 wt.%, 
P2O5 is 0.83 and 0.87wt.%, SrO is 0.02 and 0.02wt.%, BaO of 0.01 and 0.01 wt.%. It has 
higher magnesium number (Mg#) than the mafic garnet gneisses. The Magnessium number 
(Mg#) of these rocks are 60.23 and 59.33%. 
4.3.1.5  Pyroxenite. 
The pyroxenite has SiO2 content of 51 wt.%.  It has very low 7.19 alumina content relative 
lower. They have higher Fe2O3t content with 16.75 wt.% concentration. The CaO content is 
8.03 wt.%. The MgO content is high (14.1 wt.%).  Very low Na2O content of 1.31wt.%. 
K2O content of 0.21wt.%, Cr2O3 content of 0.05 wt.%, TiO2 content of 1.19 wt.%, MnO 
content of 0.27 wt.%, P2O5 content of 0.14 wt.%, SrO content of 0.02 wt.%, BaO content of 
0.01 wt%. The Magnessium number Mg# for the rock is 62.51% (Table 12). 
 
 
 
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Table 10: Major and trace elements composition of the pyroxene garnet gneiss. 
Sample S11 S15 S16A S20 S24 N58A N58h N58f N62A N63 N67 N68 
Major Elements (wt%)                     
SiO2 54.4 53.3 49.9 53.7 54.7 49.5 52.6 51.2 54.7 54.6 57.2 53.4 
TiO2 0.95 1.07 1.02 0.97 1.09 1.06 0.97 1.08 0.56 0.88 1.25 1.03 
Al2O3 19.05 18.35 19.9 18.7 18.55 19.45 19.15 19.65 18.4 17.95 23.8 18.8 
Fe2O3 7.5 8.65 9.7 7.99 9.08 10.25 8.73 8.84 7.46 8.1 2.06 8.18 
MnO 0.1 0.12 0.15 0.11 0.14 0.19 0.12 0.1 0.12 0.12 0.01 0.09 
MgO 4.33 5.04 5.61 4.93 4.7 6.65 6.15 6.48 5.72 5.64 1.18 4.98 
CaO 7.92 8.27 8.77 8.27 7.91 8.75 8.6 9.06 8.2 8.07 7.44 8.29 
Na2O 5.1 4.74 4.37 4.76 4.14 4.1 4.57 4.46 4.29 4.31 6.68 5.29 
K2O 0.56 0.37 0.26 0.49 0.51 0.29 0.35 0.32 0.51 0.53 0.3 0.53 
P2O5 0.23 0.26 0.18 0.25 0.26 0.18 0.11 0.12 0.06 0.16 0.19 0.22 
Cr2O3 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.02          0.01 0.01 
SrO 0.09 0.09 0.09 0.09 0.08 0.08 0.09 0.09 0.08 0.08 0.18 0.09 
BaO 0.03 0.02 0.01 0.02 0.03 0.01 0.01 0.01 0.02 0.02 0.01 0.02 
LOI 0.33 0.2 0.24 0.36 0.22 0.47 0.48 0.38 0.34 0.37 0.27 0.33 
Total 100.6 100.49 100.21 100.65 101.42 101 101.95 101.81 100.48 100.85 100.57 101.26 
Mg# 53.35 53.58 53.40 55.00 50.63 56.24 58.26 59.22 60.3 57.97 53.16 54.67 
Trace elements (ppm)                     
Cs 0.04 0.03 0.01 0.08 0.04 0.04 0.01 0.02 0.01 0.01 0.02 0.02 
Ba 256 170 92.3 194 227 76.2 111 87.8 197 188 106 211 
Rb 3.2 1.7 0.6 6 3.2 1 1 1 1.9 2.3 1 1.4 
Sr 709 717 747 774 636 688 729 781 681 639 1495 771 
Y 9.7 12.6 11.3 9.8 12.2 13 7.7 6.9 7.5 9.1 1.3 11 
Zr 50 39 24 34 59 25 33 33 32 42 17 45 
Nb 2.5 2.6 2.1 5.4 3.7 2.4 1.5 1.8 0.9 2 7.5 1.9 
Hf 1.3 1.2 0.8 1.1 1.6 0.8 1 0.9 0.9 1.3 0.4 1.1 
Ta 0.1 0.2 0.1 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.4 0.1 
Pb 10 2 2 2 2 2 2 2 2 2 2 2 
P 1003.86 1134.8 785.63 1091.15 1134.8 785.63 480.11 523.75 261.88 698.34 829.27 960.21 
Ti 5695.25 6414.65 6114.90 5815.15 6534.55 6354.70 5815.15 6474.60 3357.20 5275.60 7493.75 6174.85 
Th 0.19 0.05 0.05 0.18 0.25 0.05 0.1 0.05 0.12 0.2 0.05 0.07 
U 0.1 0.05 0.05 0.05 0.22 0.05 0.05 0.05 0.08 0.09 0.05 0.05 
K 4648.73 3071.48 2158.34 4067.64 4233.66 2407.38 2905.46 2656.42 4233.68 4399.69 2490.39 4399.69 
Sc 16 20 21 17 21 23 18 19 18 18 3 18 
V 195 237 242 221 231 221 230 242 175 220 90 222 
Cr 70 90 120 80 60 140 130 150 150 140 10 90 
Co 27 30 31 28 29 34 30 37 30 29 7 30 
Ni 45 54 46 48 29 78 75 90 63 67 6 50 
Cu 44 39 9 21 36 17 4 2 6 21 1 22 
Zn 81 88 84 80 87 92 97 102 78 79 19 103 
La 8.4 8 3.5 7.1 9.2 3.6 5.2 3.1 4.8 6.5 2.1 7.5 
Ce 18.8 19.6 9.3 16.7 20.6 9.8 12.8 8.9 10.1 14.2 4.7 18.7 
Pr 2.58 3 1.56 2.44 2.96 1.63 1.97 1.55 1.4 1.99 0.69 3.09 
Nd 12.2 14.4 8.9 11.4 13.5 9.2 10.2 9.1 6.7 9.5 3.5 14.7 
Sm 2.47 3.25 2.5 2.59 3.01 2.61 2.33 2.53 1.64 2.38 0.77 3.24 
Eu 1.1 1.24 1 0.99 1.18 1 0.94 0.98 0.82 0.92 0.35 1.26 
Gd 2.47 3.25 2.85 2.6 3.22 2.45 2.17 2.19 1.6 2.18 0.63 3.13 
Tb 0.34 0.41 0.39 0.33 0.43 0.37 0.3 0.27 0.22 0.3 0.07 0.39 
Dy 1.81 2.36 1.98 1.88 2.36 2.37 1.66 1.51 1.35 1.74 0.24 2.19 
Ho 0.38 0.48 0.42 0.35 0.43 0.47 0.31 0.23 0.27 0.34 0.04 0.38 
Er 1.02 1.47 1.23 1.13 1.3 1.57 0.95 0.7 0.95 1.01 0.12 1.15 
Tm 0.16 0.2 0.2 0.15 0.19 0.26 0.13 0.12 0.13 0.14 0.01 0.18 
Yb 0.96 1.04 1.04 0.94 1.06 1.54 0.77 0.67 0.73 0.88 0.05 1 
Lu 0.14 0.18 0.16 0.14 0.18 0.22 0.11 0.09 0.12 0.14 0.01 0.13 
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Table 11: Major and trace elements composition of garnet hornblende gneiss. 
Sample M2 M5A M5B M3 M4A M4B M6C 
Major Elements (wt%)           
SiO2 51.1 49.6 49.4 47.7 50.1 51.8 50.8 
TiO2 1.34 0.89 1.2 1.76 1.49 1.17 1.24 
Al2O3 19.65 19 19.3 17.9 18.15 20.3 18.75 
Fe2O3 9.26 9.68 9.83 11.9 9.65 8.62 8.64 
MnO 0.14 0.14 0.12 0.15 0.08 0.13 0.1 
CaO 8.49 8.68 8.69 9.65 9.11 8.11 9.24 
MgO 4.81 6.56 6.34 6.75 5.58 4.67 6.02 
Na2O 4.97 4.43 4.56 4.02 4.8 4.95 4.66 
K2O 0.33 0.35 0.41 0.33 0.48 0.34 0.4 
P2O5 0.41 0.15 0.3 0.49 0.43 0.4 0.22 
Cr2O3 0.01 0.02 0.02 0.02 0.01 0.01 0.02 
SrO 0.09 0.09 0.09 0.07 0.08 0.1 0.09 
BaO 0.01 0.01 0.01 0.01 0.02 0.01 0.01 
LOI 0.2 0.55 0.34 0.53 0.38 0.25 0.54 
Total 100.81 100.15 100.61 101.28 100.36 100.86 100.73 
Mg# 50.72 57.31 56.1 52.91 53.4 51.77 57.99 
Trace elements(ppm)           
Cs 0.01 0.01 0.01 0.02 0.01 0.01 0.02 
Ba 113.5 90.5 101.5 66.1 144 115.5 106 
Rb 0.6 1.3 1 0.8 1.9 0.7 1.2 
Sr 807 721 765 584 755 876 784 
Y 18.4 12.3 12.1 22.1 12.9 14.7 12.5 
Zr 113 24 40 147 60 48 33 
Nb 6.2 1.4 1.9 5 1.3 4.2 2 
Hf 2.8 0.8 1.2 3.5 1.5 1.3 1.1 
Ta 0.4 0.1 0.1 0.2 0.1 0.3 0.1 
Pb 2 2 2 2 2 2 2 
P 1789.49 654.69 1309.38 2138.65 1876.79 1745.84 960.21 
Ti 8033.3 5335.55 7194 10551.2 8932.55 7014.15 7433.8 
Th 0.09 0.05 0.1 0.23 0.09 0.14 0.18 
U 0.08 0.05 0.05 0.11 0.07 0.07 0.1 
K 2739.43 89.57 89.57 89.57 179.13 89.57 89.57 
Sc 22 22 21 28 21 18 21 
V 258 246 288 379 334 240 265 
Cr 90 150 140 140 100 70 150 
Co 29 35 35 41 34 31 33 
Ni 44 80 76 72 53 48 70 
Cu 288 11 2 186 59 132 15 
Zn 90 100 104 128 118 91 100 
La 5.7 2.8 3.3 10.3 6.1 6 5.3 
Ce 15.1 7.8 9.5 28.9 18 16 14.3 
Pr 2.53 1.48 1.84 4.87 3.19 2.64 2.58 
Nd 13.1 9.2 10.6 25.2 17.6 14 13.7 
Sm 3.58 2.51 3.11 5.76 4.62 3.57 3.82 
Eu 1.13 0.92 1.04 1.58 1.29 1.18 1.16 
Gd 4 2.71 3.16 5.79 3.95 3.78 3.49 
Tb 0.58 0.39 0.44 0.76 0.46 0.48 0.46 
Dy 3.44 2.5 2.54 4.3 2.94 2.97 2.58 
Ho 0.65 0.4 0.46 0.82 0.48 0.51 0.44 
Er 1.93 1.35 1.3 2.47 1.33 1.63 1.3 
Tm 0.32 0.19 0.17 0.36 0.2 0.24 0.17 
Yb 1.86 1.18 1.24 2.1 1.13 1.3 1.16 
Lu 0.25 0.21 0.16 0.3 0.15 0.21 0.14 
 
  
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                          Table 12: Major and trace elements composition. 
                                                      Pyroxene bearing Amphibolite                  Fault Breccia        Pyroxenite 
Sample M5C M5D N58d N58g 
Major Elements (wt%)                
SiO2 41 40.6 50.1 51 
TiO2 2.62 2.91 0.58 1.19 
Al2O3 14.1 14.35 17.35 7.19 
Fe2O3 14.65 14.8 6.99 16.75 
MnO 0.13 0.13 0.09 0.27 
MgO 11.2 10.9 2.88 14.1 
CaO 11.4 11.35 19.05 8.03 
Na2O 2.76 2.73 0.11 1.31 
K2O 0.59 0.58 0.04 0.21 
P2O5 0.83 0.87 0.08 0.14 
Cr2O3 0.03 0.02 0.01 0.05 
SrO 0.02 0.02 0.15 0.02 
BaO 0.01 0.01 0.01 0.01 
LOI 0.59 0.52 3.46 0 
Total 99.93 99.79 100.89 100.27 
Mg# 60.23 59.33 44.94 62.51 
Trace elements (ppm)       
Cs 0.01 0.01 0.06 0.05 
Ba 81.5 75.4 8.3 61.4 
Rb 1.5 1 0.5 1.9 
Sr 181 186 1275 180 
Y 15 14.3 6 16.6 
Zr 57 54 19 56 
Nb 6.6 10 3.7 1.8 
Hf 1.8 1.9 0.6 1.7 
Ta 0.3 0.5 0.1 0.1 
Pb 2 2 9 2 
P 3622.62 3797.2 349.17 611.04 
Ti 15706.9 17445.45 3477.1 7134.05 
Th 0.09 0.05 0.05 0.21 
U 0.05 0.06 0.05 0.07 
K 4897.77 4814.75 332.052 1743.27 
Sc 38 40 15 49 
V 644 604 262 394 
Cr 210 190 110 370 
Co 63 64 16 71 
Ni 132 123 51 189 
Cu 81 9 1 16 
Zn 169 178 32 179 
La 5.5 5.3 1.9 3.8 
Ce 18 17.7 4.8 11.3 
Pr 3.33 3.38 0.83 1.95 
Nd 18.8 18.8 4.2 11.2 
Sm 4.78 5.2 1.05 2.94 
Eu 1.41 1.6 0.58 0.74 
Gd 4.69 4.81 1.45 3.53 
Tb 0.6 0.62 0.17 0.53 
Dy 3.35 3.08 0.99 3.19 
Ho 0.59 0.57 0.21 0.69 
Er 1.51 1.36 0.65 1.85 
Tm 0.19 0.21 0.08 0.28 
Yb 0.87 1.04 0.61 1.69 
Lu 0.13 0.14 0.08 0.27 
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4.3.2 Trace Element Geochemistry 
The use of trace element data to characterize rocks is predicated on the assumption that, 
despite evidence for mobility of many trace and major elements during metamorphism 
(Rudnick and Presper, 1990), the preserved trace element concentrations mimic those of the 
protolith (Jahn, 1990). The rare earth elements (REEs), transition metals and other trace 
element concentrations found in the rock have been presented in Table 10, Table 11 and 
Table 12. 
4.3.2.1 Rare Earth Element (REE) 
A chondrite-normalised rare earth element (REE) diagram of the mafic garnet gneiss and its 
associated rocks were plotted together with lower crust and normal MORB (N-MORB) 
{Fig4.31). Chondrite values are from Boynton (1984). Lower crust values from Rudinick 
and Fountain (1995) and N-MORB values from Sun and McDonough (1989). The pyroxene 
garnet gneisses have very similar characteristics to the lower crust (LC). These rocks overall 
display fractionated patterns with an overall negative slope. They show moderate 
enrichment in the light rare earth rlements (LREEs) and depletion of the heavy rare earth 
elements (HREEs) which is similar to the LC with (La/Yb)N values between 1.57 and 5.89 
and (Ce/Yb)N  values between 1.62 and 5.06). The pyroxene garnet gneisses show slight 
enrichment in the LREEs compared to the middle rare earth elements (MREEs) with 
(La/Sm)N values between  0.77 and 2.13 and (Ce/Sm)N values of 0.84 - 1.83. These 
(La/Sm)N ratios show slightly elevated to nearly flat LREEs and MREEs patterns. These 
rocks again show positive Eu anomaly which resemble the LC. The Eu anomaly (Eu/Eu*) 
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ranges from 1.14 to 1.54 (Fig 4.31a). The garnet hornblende gneisses also displayed rare 
earth elements (REEs) pattern very similar to the pyroxene garnet gneisses and to the once 
exhibited by the Lower Crust (LC). They have (La/Yb)N values between 1.59 and 3.63 and 
(Ce/Yb)N  values between 1.70 and 4.12 with (La/Sm)N values between  0.66 and 1.12 and 
(Ce/Sm)N values of 0.73 - 1.21. The Eu anomaly (Eu/Eu*) range from 0.83 to 1.07 (Fig 
4.31b).  
The pyroxene bearing amphibolites display enrichment in the LREEs with slightly high 
peaks of Nd and Sm. The HREEs are depleted. They have (La/Yb)N values of 4.26 and 3.43 
and (Ce/Yb)N  values of 5.35 and 4.40. The pyroxene bearing amphibolites show nearly flat 
to negative Eu anomaly (Eu/Eu*) with values of 0.90 and 0.82.  The rocks have near similar 
characteristics to the Lower Crust (LC) (Fig 4.32c). 
The fault breccia also has the geochemical characteristics just as the mafic garnet gneisses 
and the Lower Crust (LC) but shows a Dy trough. The fault breccia even though exhibits 
similar REEs patterns as the mafic garnet gneisses and the lower crust; it is depleted in the 
REEs compared to the mafic garnet gneisses. It shows slight enrichment to nearly flat 
LREEs. HREEs are relatively slightly depleted. It has (La/Yb)N value of 2.09 and (Ce/Yb)N  
value of 2.03 with (La/Sm)N values of 1.13 and (Ce/Sm)N values of 1.10 with slight positive 
Eu Anomaly (Eu/Eu*) of 1.43 (Fig 4.32d). 
 
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The pyroxenite shows similar REEs patterns as the mafic garnet gneisses with slight 
enrichment of LREEs and nearly flat HREEs. It has (La/Yb)N value of 1.51 and  (Ce/Yb)N  
of 1.72, with slight negative Eu anomaly (Eu/Eu*) of 0.89 (Fig 4.32d). 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Figure 4.31: Chondrite-normalised plots of the REE composition of the rocks in the study 
area (a) Pyroxene garnet gneisses. (b) Garnet hornblende gneiss. With lower crust (LC) and 
Normal MORB (N-MORB). 
 
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Figure 4.32: Chondrite-normalised plots of the REE composition of the rocks in the study 
area (c) Pyroxene bearing amphibolite (d) fault breccia and pyroxenite. With lower crust 
(LC) and Normal MORB (N-MORB). 
 
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4.3.2.2 Incompatible Trace elements  
Figure 4.33 and Figure 4.34 show the incompatible trace element concentration in the rocks 
of the study area normalized to chondrite values from Thompson (1982) compared with 
Lower Crust (LC) and N-MORB. Lower Crust (LC) values are from Weaver and Tarney 
(1984) and N-MORB values from Saunder and Tarney 1984. 
The pyroxene garnet gneisses and the garnet hornblende gneisses have similar patterns as 
the Lower Crust (LC). The depletion in Rb and trough by Th shown by the mafic garnet 
gneisses are very similar to the Lower Crust (LC). The positive peak of K is also very 
similar to the Lower Crust (LC). Enrichment in the Ba and Sr shown by the mafic garnet 
gneisses are also very similar to the Lower Crust (LC). The rocks display positive peak of Sr 
(Fig 4.33A and B).  Generally, the mafic garnet gneisses display pattern similar to the Lower 
Crust (LC).  
The trace element pattern depicted by pyroxene bearing amphibolite is similar to the Lower 
Crust (LC) and the mafic garnet gneisses, though they show slight depletion in Sr and has 
positive Ce anomaly. Generally, the pyroxene bearing amphibolites show more depletion in 
the incompatible especially Rb and Th and K (Fig 4.34C) 
The fault breccia also shows more depletion of the mobile incompatible elements especially 
Ba, Rb, Th and K but show enrichment in Sr (Fig 4.34D). The pyroxenite also shows trace 
elements pattern similar to the Lower Crust (LC). 
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Figure 4.33: Chondrite normalized plots of the incompatible trace element composition of 
the rock in the study area. (A) Pyroxene garnet gneisses (B) Garnet hornblende gneisses
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Figure 4.34: Chondrite normalized plots of the incompatible trace element composition of 
the rock in the study area. (C) Pyroxene bearing amphibolite (D) Fault breccia (E) 
Pyroxenite. 
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Figure 4.35 also shows the incompatible trace element concentration in the garnet mafic 
gneisses of the study area normalized to N-MORB values from Saunder and Tarney 1984 
compared with Lower Crust (LC) and Island Arc Tholeiite (IAT). Lower Crust (LC) values 
are from Weaver and Tarney (1984) and IAT values from Attoh and Morgan (2004). 
Imprints of IAT preserved in the mafic garnet gneisses are well exhibited by the pyroxene 
garnet gneisses. (Fig 4.35A). The pyroxene garnet gneisses exhibit almost the same pattern 
as the IAT with enrichment in the Large Ion Lithophile Elements (LILE) relative to N 
MORB and the High Field Strength Elements (HFSE) expecially Sr and Ba. The LILE 
behave differently from the High Field Strength Elements (HFSE) expecially Th and Yb 
which show nearly MORB like concentrations. The pyroxene garnet gneisses generally 
behave similar to the LC with the latter relatively enriched in the LILE compared to the 
pyroxene garnet gneisses. 
The garnet hornblende gneisses also show similar pattern exhibited by the pyroxene garnet 
gneisses. (Fig 4.35B). The garnet hornblende gniesses exhibit almost the same pattern as the 
IAT though some of them exhibit depletion in K. They show very similar enrichment in the 
LILE just as the IAT compared to N MORB and then depletion in the HFSE which is nearly 
MORB like concentrations. They also behave similar to the LC with the LC relatively 
enriched in the LILE compared to the garnet hornblende gneisses. 
Generally, the mafic garnet gneisses preserve imprints of IAT which is exhibited by close 
resemblance in their trace elements. Their trace elements pattern also looks similar to the LC 
with the LC more enriched in the LILE compared to the mafic garnet gneisses. 
 
 
 
 
 
 
 
 
 
 
 
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Figure 4.35: N MORB normalized plots of the incompatible trace element composition of 
the mafic garnet gneisses. (A) Pyroxene Garnet Gneiss (B) Garnet Hornblende Gneiss. 
 
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4.3.2.3 Transition elements  
All the transition elements Scandium (Sc), Chromium (Cr), Titanium (Ti), Vanadium (V), 
Cobalt (Co), Nickel (Ni), Copper (Cu) and Zinc (Zn) are highest in the more mafic rocks. 
They are high in the pyroxenite and the pyroxene bearing amphibolite than the mafic garnet 
gneisses and the fault breccia (Tables 10 to 12). 
 These rocks are chondrite normalized with chondrite values from Langmuir et al. (1977). 
All the different rock types show similar patterns (Fig 4.36) and (Fig 4.37).  They show 
progressive depletion from Ti to Ni. They display positive Ti anomaly and negative Cr and 
Ni anomaly. Anomalies in (Ni) and (Cr) concentration may reflect the role of olivine (Ni) 
and spinel (Cr) absence in the rock. Ti anomaly may indicate the role of Fe-Ti oxides 
present in the rocks. 
 
 
 
 
 
 
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Figure 4.36: Chondrite normalized plots of transition element composition of the rocks in 
the study area. (A) Pyroxene garnet gneiss (B) Garnet hornblende gneiss. 
 
 
 
 
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Figure 4.37: Chondrite normalized plots of transition element composition of the rocks in 
the study area. (C) Pyroxene bearing amphibolite (D) Fault breccia (E) Pyroxenite. 
 
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CHAPTER FIVE 
 
5.0 DISCUSSIONS 
According to Attoh (1990), the Dahomeyan structural unit as part of the Dahomeyide forms 
during the Pan African orogeny about (900-450) Ma. The suture zone consists of mainly 
mafic garnet gneisses. These gneisses are associated with pyroxene bearing amphibolite, 
pyroxenite rocks and a brecciated rock. The structures found in the gneisses are foliations, 
ptygmatic folds, faults, joints and fractures. From field evidences, structures, 
microstructures, textures and mineralogy, indicates that the mafic garnet gneisses have 
undergone different deformational events and have had different metamorphic episodes. 
Major and trace elements also suggest Island Arc Tholeiite (IAT) protolith for these rocks.  
 
 
 
 
 
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Figure 5.1: Geological map of the area. 
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Figure 5.2: Cross section of the Study area. 
 
  
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5.1 Field Relation and Petrography 
The geology of the study area is mainly characterized by the mafic garnet gneisses, with 
pyroxene bearing amphibolite, fault breccia and pyroxenite associations. The mafic garnet 
gneisses in the area are made up of the pyroxene garnet gneisses, and garnet hornblende 
gneisses (Fig 5.1). The pyroxene garnet gneisses and the hornblende garnet gneisses have 
the same composition and are distinguished using the modal composition. They together 
form the mafic garnet gneisses characterizing the suture zone of the Pan African belt in 
Ghana. The pyroxene garnet gneisses outcrop mainly at the Naglayo hill and the Shai forest 
reserve with the garnet hornblende gneisses outcropping at the Mampong hill.  These 
gneisses are strongly sheared, deformed, sheared foliated and recrystallized. In the 
extremely sheared and deformed mafic garnet gneisses, they look homogenous with some 
parts quite heterogeneous, consisting of well-formed bands. This is due to planar and zonal 
differential shearing deformation and intense recrystallization (Kutu et al., 2014). The 
occurrence of high temperature and pressure metamorphic minerals such as pyroxenes and 
garnet indicate high intensity of metamorphism in the mafic garnet gneisses. 
The mafic garnet gneisses are generally greyish and sometimes reddish as a result of the 
occurrences of abundant garnet in the rock. The garnet crystals are millimetric to 
centimetric. The different size of the garnet crystals is probably as a result of several 
generations of garnet minerals at different conditions. The abundance of garnet crystal in the 
mafic garnet gneisses indicates high metamorphic conditions of these rocks. Some garnet 
bands observed also suggest the intensity of metamorphism in the terrane. The streaky 
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appearance characterized by prominent discontinuous layers of garnet rich and hornblende 
rich zones in some localities are interpreted as shear induced (Attoh and Nude, 2008). 
The gneissic white and dark bands and foliations found in these rocks also show that the 
mafic garnet gneisses were subjected to high degree of metamorphism. This gives 
indications of metamorphic differentiation during metamorphism where initially dispersed 
constituents segregate into layers of mafic minerals (dark compositional band) and silica 
rich (white compositional quartzo-feldspathic minerals) bands. This segregation happens as 
a result of either solution or redeposition via an aqueous phase governed by local pressure 
gradient, local melt segregation of minerals, diffusion controlled mineral development or 
segregation of minerals as a result of response to shear or stress differences (Stephens et al., 
1979). Hornblende minerals developed in the quartzofeldspathic compositional band might 
have crystallized within the quartzofelspathic layers during the period of metamorphic 
segregations. These hornblendes are well sheared into sigma and delta shape probably 
during deformation as a result of non-coaxial shearing.  Ptygmatic folds observed with the 
same composition in the area might have been formed as a result of partial melting of the 
quartzo feldspathic layers during early stages of exhumation.  
Rotation of some garnet crystal and lenticular pyroxenite boudin observed in the gneisses 
may have formed as a result of non-coaxial or simple shearing during metamorphism. 
Pressure shadow of quartz observed around large garnet porphyroblast is formed during 
metamorphism where quartz is dissolved from the high stress (generally in the direction of 
the highest stress) and reprecipated in the low stress area adjacent to the porphyroblastic 
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garnet during metamorphism. Pressure shadow gives indication of direction of shearing 
during metamorphism. The halos of feldspar mostly observed around garnet suggest 
chemical reaction between garnet and pyroxene during metamorphism. The sigma and delta 
developed hornblende within the quartzo-feldspathic layers, together with asymmetrical 
pressure shadows around quartz form spectacular sets of kinematic indicators, which 
indicate SSW-directed thrusting of the suture zone rocks onto the West African Craton 
(WAC) margin (Attoh and Nude, 2008). 
Under the thin section, the porphyroblastic nature of garnet, clinopyroxene and some 
plagioclase oriented in a granoblastic to granomatoblastic groundmass of plagioclase and 
quartz indicates high degree of metamorphism the rocks may have undergone. The 
porphyroblastic garnet and clinopyroxene minerals that are highly fractured, lenticular and 
mostly appear broken and are surrounded the S1 external foliation are pre kinematic 
minerals; they formed prior to the formation of the S1 foliation. The broken and highly 
cracked nature displayed by the clinopyroxenes indicates high degree of deformation they 
have undergone as a result of the later deformation experienced by the rocks. The broken 
nature of these minerals indicates the intensity of deformation the rocks have been subjected 
to. The external S1 wrapped or compressed around the pre-kinematic porphyroblastic garnet 
and clinopyroxene is as a result of flattening in the matrix. The inclusions of hornblende and 
ilmenite in the porphyroblastic garnet and clinopyroxene show prograde metamorphism. 
They indicate that the hornblende and the ilmenite were formed prior to the formation of the 
porphyroblastic garnet and clinopyroxenes. The porphyroblastic garnet and clinopyroxenes 
might have enveloped the hornblende and ilmenite as they grow or evolve from them.  
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A porphyroblast can include a mineral that becomes a reactant in a later metamorphic 
reaction (Winter, 2001). This indicates that the hornblende inclusions may have formed the 
garnet and the clinopyroxenes. Furthermore, rims of hornblende and chlorite around these 
porphyroblast are as a result of retrograde metamorphism which there is introduction of 
water which alters them into low grade mineral. The corona texture of clinopyroxene 
surrounds porphyroblastic garnet observed in the thin section also indicates a replacement 
and reaction texture which did not react to completion (Winter, 2001). This may have 
occurred during retrogressive metamorphism in very early stages of exhumation. 
Replacement textures form during retrograde metamorphism (Winter, 2001). 
Pre-kinematic and syn to post-kinematic garnets observed suggest different metamorphic 
episodes within the magic garnetiferous gneisses. This implies that the rocks were subjected 
to at least transitional states facies metamorphism. The pre-kinematic garnets may have 
formed synchronously with the pre-kinematic porphyrobalstic clinopyroxene and hence 
exhibit similar deformational and metamorphic features. They are broken with 
intracrystalline fractures, have anhedral shape and wrapped by S1 foliation. They form prior 
to the formation of the later deformational and metamorphic events. The syn to post 
porphyroblastic garnet are less deformed relative to the pre-kinematic types and have well-
formed shape compared to the prekinematic ones.  They are the abundant garnet generation 
in the mafic garnet gneisses. Inclusions of pre-kinematic clinopyroxene in the core of these 
garnets indicate they formed after the clinopyroxenes had formed. 
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The presence of garnet megacrysts with ex-solved rods of rutile indicate ultrahigh pressure 
metamorphism as titanium is stable as a solid solution in garnet under high pressure 
(>3.0Gpa). This indicates that the rocks were subducted to mantle depths (Attoh and Nude 
2008). Not only do the exsolution rods of rutile provide evidence of subduction and 
ultrahigh pressure metamorphism but they also provide evidence of exhumation and cooling 
as they occur when a Ti rich precursor precipitates excess Ti in response to decreasing Ti 
solubility during cooling or decompression (Zhang et al., 2003).  
Plagioclase and quartz constitute the leucocratic and granoblastic groundmass in these mafic 
garnet gneisses. The plagioclase porphyroblast with bent twinning polysynthetic flexural 
twins and spectacularly abnormal extinction indicates deformations they have undergone by 
later deformational event. The yellow tint displayed by some plagioclase observed in the 
rocks is as a result of retrograde metamorphism of plagioclase into epidotes during late 
exhumation stage where there is influx of water. The undulatory extinctions displayed by 
quartz and plagioclase are formed as a result of strain and provide even more subtle 
evidence of metamorphism and deformation. They are formed probably by recovery and 
recrystallization during deformation. Sutured boundaries of quartz and some plagioclase also 
indicate recrystallization during deformations and metamorphism. Rutile is generally 
regarded as a characteristic constituent of basic rocks belonging to the eclogite and granulite 
facies (Knorring and Kennedy, 1958). 
From the mineralogy of the rocks, it can be suggested that the rocks were formed from a 
basic or mafic protolith, possibly rocks of basaltic composition. The gneisses can therefore 
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be said to have formed from the subduction of the roots of an island arc (Attoh and Nude, 
2008) to mantle depths where it was subjected to ultrahigh pressure and high pressure 
metamorphism and deformation by the collision between the West African Craton and 
exotic blocks from the east during the assemblage of NW Gondwana and subsequent 
exhumation. 
The pyroxene bearing amphibolites are observed at the Eastern Quarries at the Mampong 
Hill. They are tectonically welded within the mafic garnet gneisses. These rocks are strongly 
deformed and sheared together with mafic garnet gneisses with gradational contact with the 
mafic garnet gneisses. The pyroxene bearing amphibolites are fine to medium grained dark 
rocks. The dark color of the rocks is as result of predominant mafic minerals present in the 
rocks with some plagioclase. 
Under the thin section, the rock is predominantly made up of hornblende. They contain 
plagioclase with clinopyroxene and garnet. The presence of clinopyroxene and garnet 
indicates metamorphism at relatively higher temperature and pressure conditions and 
confirms that these rocks may have metamorphosed into the mafic garnet gneisses under 
higher temperature and pressure. The garnets have inclusion of hornblende and indicate 
prograde metamorphism. These rocks might have prograded into the mafic garnet gneisses 
under higher pressure and temperatures where most of the hornblende metamorphosed into 
garnet and clinopyroxenes and Na rich plagioclase into more Ca rich plagioclase.  
The pyroxenites associated with mafic garnet gneisses may probably be older materials 
thrusted into gneisses. Some boudins formed from these pyroxenites indicate that these 
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pyroxenites may be older than the mafic garnet gneisses. They are thrust along the foliations 
in the mafic garnet gneisses. These rocks are fine to medium grained dark colored rock. 
These pyroxenites are also tectonically deformed together with the mafic garnet gneisses. 
Under thin section, the pyroxenite is made up of variable proportions of orthopyroxenes and 
clinopyroxenes. These mineral show highly intracrystalline fractures and mostly with 
anhedral shapes. This indicates deformation the pyroxenite may have undergone. 
The brecciated rock found within the mafic garnet gneisses at PW quarries were formed as a 
result of cataclasis and crushing in fractures within the gneisses as a result of brittle 
deformation at relatively lower temperature and pressure. Microscopic observation of the 
fault breccia indicates a mortar texture of porphyroclast of quartz surrounded by crushed 
matrix derived by cataclasis and recrystallization. It indicates that the quartz minerals are 
pre-kinematic to the cataclasis. 
5.2 Metamorphism 
On the basis of microstructures, mineral assemblages and reaction relations between mineral 
phases, at least four metamorphic episodes can be established for the mafic garnet gneisses 
in the area. It encompasses early prograde metamorphism to peak granulite facies stage or 
episode and then later retrogression metamorphic stages. The early prograde metamorphism 
is preserved only as inclusion of prograde hornblende and ilmenite within porphyroblastic 
garnet and clinopyroxenes. These textural features suggest that the mafic garnet gneisses 
have a prograde history prior to the peak granulite metamorphism. 
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Peak metamorphism is represented by granulite facies assemblage. The granulite stage is 
characterized by the deformed porphyroblastic garnet (Grt), clinopyroxene (Cpx) and some 
plagioclase (Pl) and quartz mineral assemblages (Cpx + Grt + Pl + Qtz).  
The next metamorphic stage is the hornblende-granulite stage and characterized by 
crystallizations of porphyroblast garnet (with inclusions of rutile exsolusion, ilmenite and 
earlier formed clinopyroxenes), recrystallization of plagioclase (Pl) and quartz (Qtz), 
development of dark reddish brown hornblende (Hbl) and corona texture of clinopyroxene 
(Cpx) around garnet. This stage is characterized by probable early stages of exhumation. 
The crystallization of green hornblende rims around some porphyroblastic clinopyroxenes 
and garnet characterize retrogressive amphibolite stage during exhumation via extensional 
decompression. Crystallization of green fibrous chlorite around clinopyroxene and garnet, 
and alteration of plagioclase to epidote define retrograde greenschist facies metamorphic 
stage. This might have happened as a result of influx of water near the earth surface. 
5.3 Deformations, Structures and Kinematics 
The area has undergone several regimes of tectonic and metamorphic events, thus, most of 
the rocks show numerous deformational structures. Different deformational events 
characterize the mafic garnet gneisses in the suture zone. The structures observed in the 
study area are foliation and banding, ptygmatic folds, faults, fractures joints and boudins. 
This implies that the rocks have undergone both brittle and ductile deformational phases. 
Three main deformational events may have occurred in the suture from its prograde history 
to the later successive retrograde episodes. 
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5.3.1 D1 Deformation 
The gneissic banding observed in the gneisses indicates the gneisses were subjected to high 
temperature and pressure regimes that segregated the felsic and mafic minerals into bands. 
This gives indications of metamorphic differentiation during metamorphism where initially 
dispersed constituents segregate into layers of mafic minerals (dark compositional band) and 
silica rich (white compositional quartzo-feldspathic minerals) bands. The general trend of 
these structures is NE-SW direction with few having NNE-SSW direction with gentle dips 
towards SE. This may imply that the direction of maximum strain that formed the foliations 
acted perpendicularly to the general NE-SW trend. These are formed during compression 
with the maximum NW/SE stress causing the rocks to strain or deformed in the NE/SW 
direction. Deformation of the terrane and the rocks was intense and by compressional 
thrusting and shearing with the direction of tectonic transports from the SE to NW in a 
craton-ward vergence on subparallel thrust planes (Kutu et al., 2014).  
The stereo plot, confirms that the foliations in the mafic garnet gneisses have predominantly 
southeasterly dip direction with gentle dips. The contoured poles–density plot indicated that 
majority of the poles are concentrated close to the center which shows that the foliations in 
the area is of gentle dip. These foliations are therefore S1 fabric structures in the area. 
The sigma and delta shaped hornblende within the quartzofeldspathic compositional bands 
represent L1 fabric mineral lineation in the rock. They give indications of simple non coaxial 
shearing in the area during D1 deformation. Structural attitudes indicated that the L1 mineral 
lineations have moderate average plunge of 150 mainly toward SW and NE also indicating 
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that the direction of maximum shearing direction is from the NW/SE. The pole plot indicates 
NE/SW trend of the mineral lineations with gently plunge of NE mainly. 
The older pyroxenite materials were thrusted into the mafic garnet gneisses during the main 
D1 deformation. The boudins formed as a response to layer parallel extension or layer 
perpendicular flattening of the stiff pyroxenite materials. They stretch as a result of ductile 
contrast of less competent participating layers or rocks. 
The gneissic bands, foliations and the mineral lineations in the rocks are characterized by D1 
deformational event. The D1 deformational event produced the S1 and the L1 structural fabric 
in the rocks. A simple or non-coaxial shearing characterizes this deformational event. Shear 
sense indicators during D1 deformational events are mainly dextral and are represented by 
sheared pyroxenite boudin within the mafic garnet gneiss (Fig 4.24), clockwise rotation of 
garnet minerals (Fig 4.25) and the stretched hornblende mineral lineation into sigma and 
delta structure within the felsic compositional bands (Fig 4.22). This indicates that rock have 
undergone a non-coaxial shearing during D1 deformation. The foliations and mineral 
lineation characterizing the D1 deformational event indicates that the deformation is a 
ductile deformation which occurred in high temperature regime. 
 
5.3.2 D2 Deformation 
The ptygmatic fold observed clearly crosscut the foliations. It shows that they postdate the 
foliations in the rocks. The ptygmatic folds have the same composition as the felsic 
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compositional bands within the rocks. Figure 4.3b indicates a felsic compositional bands 
folded into ptygmatic folds. I therefore suggest most of the folding with composition as the 
felsic compositional bands is ptygmatic folds, formed as a result of partial melting of the 
felsic bands in the rocks within the rock. They might have been formed as a result of 
decompression during early stages of exhumation of the mafic garnet gneisses in which 
temperature might have remained relatively constant with decreasing pressure and addition 
of some volatiles. During this stage, the felsic bands with relatively lower melting point and 
viscous may have experienced decompressional melting, which then might have melted and 
begin flowing within the mafic garnet gneisses. The mafic bands may have not melted at this 
stage because of the higher melting point of mafic minerals. These ptygmatic folds 
characterize the D2 deformational event in the area.  
Upon axial measurements of folds axis, it was observed that they are of different 
orientations. Some have NE/SW trend and others with NW/SE trends with average of 25o 
plunge. From the stereographic poles plot these folds trend SW/NE and SE/NW with 
average plunge of 150 SW and SE respectively indicating flow in different directions of the 
melted felsic compositional bands. Deformation at this stage is ductile deformation. 
5.3.3 D3 Deformation 
The last stage of deformation D3 is characterized by the development of brittle deformational 
structures. They are faults, fractures and joints. The brittle faults (Fig 4.3c) crosscut the D1 
and D2 deformational structures. This indicates that the brittle deformational event postdate 
the ductile deformation in the area. It gives indications that the brittle deformations are 
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youngest among the deformation in the area. Deformation moves from ductile to brittle as a 
result of exhumation and subsequent retrogressive metamorphism. The changing of 
deformation from ductile to brittle indicates prograde metamorphism and then later 
retrogressive metamorphism. Deformation becomes brittle as a result of decompression and 
exhumation where the rocks get closer to the surface with temperature decrease. These faults 
seen mainly in the area are normal faulting where the hanging wall have moved down 
relatively to the footwall as a result of extension during decompression. This gives 
indication of crustal extension during exhumation. 
These normal dip slip faults have moderate dips with two different direction of dip. These 
faults mainly dip SW and strike NW/SE with few having a NW dip direction and NE/SW 
strike.  The predominant NW/SE strike of the faults indicates extension in the NW/SE and 
compression in NE/SW direction during extension. This shows that direction of maximum 
stress during compression experiences maximum extension during dilation or extension. 
From the stereographic pole plots, it can be inferred that the faults have moderate dips with 
average of about 350 to 400. 
The thrusted rigid older materials (pyroxenite) within the mafic garnet gneiss are fractured 
and exhibit dextral displaced grains but have overall sinistral sense of shearing during D3 
deformation (Fig 4.29). 
In summary, the Shai Hills gneisses have undergone series of deformational and 
metamorphic events. They have undergone prograde metamorphism as a result of the 
subduction to the granulitic facies which have clinopyroxenes and garnets as stable mineral 
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assemblages at high temperature and pressure. Textural evidence is the inclusions of 
hornblende and ilmenite, in porphyroblastic clinopyroxene and garnet. Exhumations of these 
rocks occurred as a result of decompression and went through to the amphibolite facies and 
to greenschist facies during retrograde metamorphism 
 
5.4 Geochemistry 
5.4.1 Classification 
Classifications diagrams by Le Bas et al. (1986), and Cox et al. (1979), were plotted (Fig 
5.3a and Fig 5.3b) for the mafic garnet gneisses in the study area. According to Le Bas et al, 
(1986) classification, the pyroxene garnet gneisses plot as basalts, basaltic andesites, basaltic 
trachy andesites and trachy andesites. The garnet hornblende gneisses plot in basalt and 
trachy basalt fields. The fault breccia is classified as basalt. In the Fig 5.3 b, the pyroxene 
garnet gneisses plot in basalt, basaltic andesite and trachy andesite field with garnet 
hornblende gneisses ploting in the basalt field.  
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Figure 5.3: Geochemical classifications of rocks in the study area using TAS diagram (a) Le Bas et 
al., (1986) (b) Cox et al., (1979). 
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According to Cox et al. (1979)’s scheme, the mafic garnet gneisses mostly classify as 
subalkaline/tholeiitic series rocks. On the classification plot of SiO2 versus K2O of Peccerillo 
and Taylor., (1976) (Fig 5.4), the rocks plot mainly in the low K tholeiitic series. Almost all 
the pyroxene garnet gneisses plot in the low K tholeiitic field. The garnet hornblende 
gneisses plot also in the low K tholeiitic fields with few fallling in the calc alkaline series 
field. 
                                              
 
Figure 5.4: Classification plot of SiO2 Versus K2O (after Peccerillo and Taylor, 1974). 
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5.4.2 Tectonic Setting 
The tectonic settings of the mafic garnet gneisses in the study area have been discussed 
using mostly immobile elements and their ratios and some major elements. Numerous 
discrimination diagrams have been plotted for the mafic garnet gneisses and its associated 
rocks. This work thus mainly relied on high field strength immobile elements such as Ti, Zr, 
Y, Nb, and immobile transition elements such Cr and some major elements in discussing the 
tectonic setting. These immobile elements may be less affected by metamorphic processes 
compared to the mobile trace elements. Furthermore, despite evidence for mobility of many 
trace and major elements during metamorphism (Rudnick and Presper, 1990), the preserved 
trace element concentrations mimic those of the protolith (Jahn, 1990). 
Figure 5.5 shows a Cr versus Y plot of these mafic garnet gneisses to distinguish basalts 
with N-MORB compositions from those of island arc tholeiite affinities (Pearce et al., 
1984).  The Cr and Y discriminates effectively between MORB and volcanic arc basalts, 
with small amount of overlap between the two fields.  Y is more depleted in volcanic basalts 
relative to other basalt. The mafic garnet gneisses fall broadly in the field of Island Arc 
tholeites (IAT). They showed depletion in Y with respect to MORB but with wide range of 
Cr contents.  
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Figure 5.5: Tectonic discrimination diagram Cr vs Y plot (Fields for MORB and IAT after 
Pearce et al., 1984) 
 
Pearce and Can (1973) proposed the Ti-Zr diagram which can be used to infer the tectonic 
setting of basalts. The diagram has four fields. Field A contains island arc tholeiites, C 
contains calc alkaline basalts and D contains MORB. Field B contains all three types (Fig 
5.6). Most of the pyroxene garnet gneisses plotted in field A, the garnet hornblende gneisses 
plotted within Field A, B with two samples plotted within Field D.  
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Figure 5.6: Tectonic discrimination diagram of Ti vs Zr plot (Pearce and Can, 1973) 
 
Pearce et al., (1977), proposed MgO-FeO-Al2O3 discriminatory tectonic diagram to 
distinguish between different basalts from different tectonic environments. It applies to 
subalkaline basalts and basaltic andesites. This diagram discriminates between the following 
tectonic environments; ocean ridge basalt and floor basalts (MORB), ocean island basalts, 
continental basalts, volcanic arc basalts and active continental margin basalts and island arc 
basalts, and spreading centre basalts (Fig 5.7). Almost all the samples plotted in the island 
arc and active continental margin that is the orogenic field. 
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Figure 5.7: Tectonic discrimination diagram of MgO-FeO-Al2O3 (Pearce et al., 1977). 
 
Although major elements discriminant diagrams are proposed for relatively unaltered 
basaltic rocks, it is used here to compare the concentrations of these elements in the mafic 
garnet gneisses with those estimated for basaltic rocks formed in various tectonic 
environments 
Mullen (1983) proposed a discriminatory diagram for Basalt and basaltic andesites with 
silica content from 45 to 54 wt% using MnO2, TiO2 and P2O5 concentrations. This diagram 
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discriminates the following; MORB, ocean island tholeiites; ocean island alkali basalts; 
isaland arc tholeiites; calc alkaline basalts (Fig 5.8). 
The samples plotted mainly in the island arc basalts fields and the ocean island alkali basalts 
(OIA) with few in the MORB fields. 
 
Figure 5.8: Tectonic discrimination diagram of MnO2, TiO2 and P2O5 (Mullen, 1983) 
 
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From the REEs plots (Fig 4.31 and Fig 4.32), generally the rocks display pattern similar to 
the Lower Crust (LC) with enrichment of light rare earth elements (LREEs) and relative 
depletion in the heavy rare earth elements (HREEs). Rudnik and Fountain (1995) proposed 
enrichment of LREEs and small positive europium anomaly for Lower Crust (LC).  All the 
pyroxene garnet gneisses have positive anomaly of europium. These pyroxene garnet 
gneisses also display enrichment in the LREEs and have depleted Heavy HREEs. They 
show very similar pattern to the lower crust (LC).  
The garnet hornblende gneisses display similar trend displayed by the pyroxene garnet 
gneisses and the Lower Crust. They have slight enrichment of LREEs and depleted HREEs 
with positive europium anomaly. Few samples displayed almost flat to negative europium 
anomalies. 
The pyroxene bearing amphibolite also displayed similar pattern to the Lower Crust (LC).  It 
displayed apparent enrichment in the LREEs. Ce, Nd and Sm yielding a relatively flat 
(La/Sm)N with slight negative to flat Eu anomaly.  
The fault rock or breccia displayed similar pattern as the pyroxene garnet gneisses and the 
Lower Crust (LC). It has slight enrichment of LREEs relative to the HREEs with very 
pronounced positive Eu anomaly. Even though pattern resembles Lower Crust (LC); the rare 
earth elements (REEs) of the Fault Breccia are more depleted relative to the Lower Crust 
(LC) and the mafic garnet gneisses.  
Humphries (1984) showed that there is no simple relationship between degrees of mobility 
of REEs and the rock or metamorphic grade and emphasizes the mineralogical and fluid 
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control. REEs may be mobilized by halogen rich or carbonate rich mineralizing fluids in the 
rock in which they would otherwise be stable with respect to the movement of an aqueous 
fluid. This is to say even though REEs are immobile, they can become mobile in certain 
fluids such as carbonate rich fluids.  
The pyroxenite displays almost flat pattern of REEs with negative Eu anomaly. It has 
slightly enrichment in the LREEs with slightly higher Ce, Nd and Sm. 
The incompatible trace elements of the mafic garnet gneisses also show similar pattern in 
Lower Crust (LC). Rudnik and Fountain (1995), proposed that the Lower Crust (LC) is 
depleted in Rb, Th, K and Pb. The pyroxene garnet gneisses and garnet hornblende gneisses 
display similar pattern to the Lower Crust (LC). The positive peak of Sr observed may be 
controlled by plagioclase in the rocks. Sr substitutes for Ca in the plagioclase. The pyroxene 
bearing amphibolites show pattern almost similar to the mafic garnet gneisses and the Lower 
Crust (LC).  
The slight Ti peaks in the garnet hornblende gneisses, the pyroxene garnet gneisses and the 
pyroxene bearing amphibolite might be as a result of accessory opaque phases such as rutile 
and ilmenite in them. The fault rock also displays similar pattern to the Lower Crust (LC) 
and the mafic garnet gneisses, but is more depleted in the incompatibles such as Ba, Rb, Th 
and K due to fluid activities within the fault breaks since these elements are more mobile. 
Even though Sr is also mobile, it displays high positive peaks and enriched. This is due to 
the substitution of a Sr for Ca in plagioclase. 
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General enrichment in the LILE in the mafic garnet gneisses compared to N MORB and 
HFSE can be as a result of participation of H2O rich fluids in the genesis of subduction zone 
magmas. These fluids are enriched LILE from the crust. 
5.4.3 Petrogenesis 
5.4.3.1 Magma type 
The origin and the magmatic type of the Shai Hill gneisses from the Dahomeyide belt, 
Southeastern Ghana have been evaluated using the major and trace elements compositions 
and discussed in this section. 
Several criteria have been used to evaluate the composition of primary magmas. Some of 
these criteria include low SiO2, high magnessium number (Mg#) and low alkalis among 
others. According to Winter (2001), primary magmas have Mg# > 65, Cr > 1000 ppm and 
Ni > 400 – 500 ppm. The mafic garnet gneisses have magnessium number (Mg#) between 
50.63 and 60.30. These suggest the rocks are not primary magmas and that they are evolved 
magmas in which their parent magmas may have experienced some degree of fractionation. 
Their Cr content ranges from 10 ppm to 150 ppm and Ni content of 6 ppm to 90 ppm. This 
shows that the magma that formed the mafic garnet gneisses are not primary magmas and 
probably might have undergone some degree of fractionation. These rocks also have 
relatively high SiO2 content (Table 10 and Table 11). The fault breccia has relatively low 
magnessium number (Mg#), low Ni and Cr content and have high SiO2 (Table 12), also 
giving indications of fractionation. 
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The magnessium number (Mg#), the Cr, Ni and the SiO2 contents of the pyroxene bearing 
amphibolite (Table 12), also suggests that the rocks are not from primary magmas. The 
magmas have undergone fractionation during ascent of the magma and subsequent 
subduction. The pyroxene bearing amphibolites have relatively higher magnessium number 
(Mg#) and higher Cr and Ni content with low SiO2 compared to the mafic garnet gneisses. 
This indicate that the mafic garnet gneisses are more evolved than the pyroxene bearing 
amphibolite.  The pyroxenite has the highest magnessium number (Mg#) of 62.51 and has 
high Ni and Cr content relative to the mafic garnet gneisses. These indicate some level of 
fractionation but less evolved relative to the mafic garnet gneisses. 
The evolution of these rocks were further analyzed using the Irvine and Baragar (1971), 
AFM diagram (Fig 5.9). In the diagram, the mafic garnet gneisses (i.e, pyroxene garnet 
gneisses and the garnet hornblende gneisses) plot closer to the FeO and the total alkali end, 
away from the MgO end. This suggests that these mafic garnet gneisses were 
metamorphosed from evolved magmas. One sample of the pyroxene garnet gneisses plots 
very close to the total alkali giving indication of high degree of fractionation. The fault 
breccia also plots closer to the FeO and further away from the MgO ends showing higher 
degree of fractionation. Pyroxene bearing amphibolite and the pyroxenite plot relatively 
closer to the MgO and suggest lesser degree of fractionation relative to the mafic garnet 
gneisses. 
 
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Figure 5.9: AFM diagram showing samples from the study area (After Irvine and Barager, 
1971). 
  
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5.4.3.2 Magmatic Protolith 
From the major elements composition, the mafic garnet gneisses are mainly basaltic and 
basaltic andesite with N67 exhibiting andesite composition. The association of basaltic, 
basaltic andesite and andesite rocks indicates an arc setting. The mafic garnet gneisses 
averagely display high Al2O3 content with low K2O (Table 10 and 11). High Al2O3 is 
particularly common in association with calc alkaline magmas which is an arc tholeiite 
signature (Winter 2001). According to Winter (2001), high alumina basalts are parental to 
the calc alkaline arc series. The rocks are tholeiite and have arc characteristics. From Fig 
5.2, the rocks plot mainly in the tholeiite series and few in the calc alkaline series with 
mainly low K to medium K series showing arc signatures.   
The association of calc alkaline and tholeiites magmas are restricted to an arc setting 
(Winter 2001) which indicate that the protolith of these mafic garnet gneisses are from arc 
setting. Attoh and Morgan (2004) suggest that these mafic garnet gneisses have preserved 
the geochemical imprints of their magmatic protoliths display rare earth element (REE) and 
trace element patterns quite similar to mafic lower crust composition. They concluded that 
the High Pressure (HP) granulites were derived from two types of tholeiitic magmas: 
dominantly island arc tholeiites (IAT) together with subordinate N-MORB tholeiites with 
the island arc tholeiites (IAT) showing peculiar LREE enrichment with flat (La/Yb)N 
patterns where the N-MORB are slightly depleted in LREE and have low (La/Yb)N relative 
to the lower crust. Compatible element concentration for the more primitive island arc 
basalts have Ni values of 75 to 150 ppm, Cr and V content of 200 to 400 ppm (Winter 
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2001). From tables 10 and 11, the mafic garnet gneisses are characterized by very low Ni 
content and Cr and V compared to these values. This indicates that the basaltic protolith of 
these gneisses are not from primary magma but rather resulted from partial melting and 
fractionation of the primary magma.  
From the REE plot (Fig 4.31), the mafic garnet gneisses display slightly enriched LREE 
relatively to the HREE. REE trace element data from the mafic garnet gneisses exhibit 
island arc with flat to LREE enriched patterns with (La/Yb)N ratios of 1.57 to 5.90.  This can 
be due to low degree of partial melting of a primitive mantle (Winter 2001) or the mantle 
source for these island arc magmas is a heterogeneous mixture of depleted MORB and 
enriched Ocean Island Basalt (OIB) mantle sources giving it different incompatible element 
concentrations, reflecting variable enrichment and depletion (Thompson et.al., 1984). The 
HREE pattern is relatively flat and implies that garnet, which partitions among the HREE, 
may not have been a major phase during the generation of these magmas.  
Again from the incompatible trace elements plots, the mafic garnet gneisses display slight 
enrichment in large ion lithophile elements (LILE) relative to the high field strength element 
(HFSE) with slight negative Nb and Ta anomaly. Also the HFSE are slightly depleted 
compared to N-MORB.  The enrichment in the LILE is explained by the participation of 
water rich fluids in the genesis of subducted slab (Winter, 2001). The fluid from the 
subducted slab enriched in the mobile LILE lower the melting temperature of the solid rocks 
and concentrate LILE in the resulting hydrated magma. This may be the case for the 
generation of the mafic garnet gneisses. The lack of negative Yb anomaly suggests that the 
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magma source is not deep (Winter, 2001). Furthermore, the relatively flat HFSE pattern 
observed approximately one (1), means the concentration of these elements is similar to that 
of N-MORB and suggest the source of island arc magmas is similar to MORB which is from 
the depleted mantle but not the subducted slab (Winter, 2001). 
McCulloch and Gamble (1991) concluded that the immobile HFSE concentrations are 
similar to those of MORB, and probably reflect overall mantle source characteristics, 
whereas the LILE reflect the more water soluble components from the slab. The enrichment 
in LREE is a feature of calc-alkaline suites (Donnelly and Rogers, 1978). In contrast, typical 
Island Arc Tholeiites (IATs) usually show nearly flat REE patterns. The samples display 
averagely lower Ti (Table 10 and 11) values showing Island Arc Tholeiites (IAT) 
characteristics with few having relatively high N-MORB Ti character.   
In summary, the LILE enriched relative to the HFSE and the flat to slightly enriched LREEs 
compared to HREE is typical of Island arc magma origin. These mafic garnet gneisses 
exhibit geochemical patterns similarto LC. 
 
 
 
 
 
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Proposed Model: Formation and Subduction of the Shai hills Suture rocks 
The history of the rocks in the study area can be traced back to the breakup of Rodinia and 
the rifting associated with the event about 750 million years ago. Ocean closure and 
subduction of ocean crust lithologies to lower crustal and mantle depths during the 
assemblage of NW Gondwana led to high and ultrahigh pressure metamorphism which is 
likely to have steered to the formation of the mafic garnet gneisses since the gneisses are 
said to preserve signatures of island arc tholeiites and subordinate imprints of N-MORBs 
(Attoh and Morgan, 2004). The mafic garnet gneisses (granulite) contain tholeiitic lavas 
formed in an oceanic island-arc environment. The mafic garnet gneisses experienced a 
different pressure-temperature-time (P-T-t) path during subduction as a result collision 
between the West African Craton (WAC) and group of eastern plates. Peak metamorphic 
conditions for the shai hill suture zone were reached at granulite facies and then later 
exhumation. This model for the subduction and exhumation of the shai hill suture zone is 
based on the metamorphic and deformation constraints.  Fig 5.10 is a proposed geodynamic 
model for the evolution of Suture zone rocks in Ghana modified after Attoh and Nude 
(2008). 
Stage A represents a stable West African Craton (WAC). Stage B represents initial rifting 
and continental breakout. This may be a result of plume of magma rising up from within the 
mantle. The Plume ponds at the base of the continent (WAC) and may have resulted into 
thermal swelling of the WAC and might have produce a broad dome which was then 
followed by normal faulting and rifting. The lithospheric breakup of the WAC resulted in 
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the formation of ocean basin on the eastern margin of the WAC represented in stage C. The 
granulite stage characterized by a pressure increase with relatively temperature decrease is 
related to the burial of the oceanic crust and part of the WAC within a subduction zone or 
during collisional processes. Stages D, E and F represent collision of the WAC and group of 
eastern plates to close up the ocean basin which might have created a subduction zone. The 
ocean crust break at some point and descended together with part of the WAC into lower 
crust to mantle depth. This resulted into the prograde metamorphism recorded in the mafic 
gniesses. This actually brought about the D1 deformation and the granulite metamorphism at 
peak collision where subduction ceased.  
The later retrograde P–T evolution may occur during exhumation. This resulted into the D2 
and the D3 deformations and the hornblende-granulite to the greenschist metamorphism. 
This is represented in Stage G. 
 
 
 
 
 
 
 
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Figure 5.10: Proposed model for the Pan African suture zone in Ghana based on Wilson 
cycle reconstructions. (Modified after Attoh and Nude, 2008). 
 
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CHAPTER SIX 
6.0 CONCLUSIONS AND RECOMMENDATIONS 
6.1       Conclusions 
The petrological and structural studies of the mafic garnet gneisses at the Shai Hills suture 
zone indicate that the suture rocks have undergone solid state poly deformational and poly 
metamorphic events and episodes as a result of subduction and later exhumation. Indications 
are that, these mafic garnet gneisses, were subducted under increased pressure and 
temperature conditions, and went through prograde metamorphism. Even though evidences 
of prograde metamorphic imprints are rarely seen, the observation of hornblende and 
ilmenite in stable mineral assemblages of porphyroblastic garnet and clinopyroxene at peak 
granulitic stage indicates prograde metamorphism. This suggests, the rocks were subjected 
to prograde metamorphism during subduction before their exhumation. Peak metamorphism 
is defined by granulitic metamorphic assemblages of garnet, clinopyroxene, plagioclase and 
some quartz. This was immediately followed by hornblende-granulite facies assemblages as 
a result of very early stages of exhumation. This stage was characterized by the development 
or crystallization of dark reddish hornblende in the rock and a corona texture of 
clinopyroxenes around garnet.  
The amphibolite and the greenschist facies observed in the rocks define major retrogressive 
metamorphism as a result of exhumation accompanied by a decrease in temperature and 
pressure conditions, as well as possible influx of water. The amphibolite facies followed the 
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hornblende-granulite with the introduction of green hornblendes around the rims of some 
clinopyroxenes and garnet. Greenschist facies which follows the amphibolite facies in the 
retrogressive path is characterized by development of chlorite mineral around the rims of 
garnet and some clinopyroxenes, also the introduction of epidotes from the alteration of 
plagioclase. This indicates that the rocks have undergone polymetamophic episodes.  
From textural and structural evidence, it can be concluded that the rocks have undergone 
intense deformation. The rocks have undergone at least three deformational events. This can 
be seen from various structures observed in the area. The rocks were subjected to ductile 
deformation as a result of subduction. The development of foliations and gneissic bands 
indicate ductile deformation.  
The first deformational event is characterized by the development of gneissic banding, 
foliations, and boudinage and mineral lineations in the rocks. The D1 deformations is 
characterized by dextral non-axial shearing or simple shearing evidenced by the rotations of 
some garnet porphyroblasts, hornblende mineral lineation in the felsic compositional bands 
which developed into sigma and delta structures and stretched and lenticular pyroxenite 
boudins within the mafic garnet gneisses.  
The D2 deformation is also characterized by ptygmatic folds found in the area. The 
ptygmatic folds postdate the gneissic bands and foliations in the mafic garnet gneisses and 
hence are D2 deformational event. The ptygmatic folds were formed from partial melting of 
the felsic compositional bands as a result of decompression during exhumation where 
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conditions must have been conducive for the felsic bands to melt. Conditions such as 
decrease pressure at relative constant temperature and with the introduction of fluids. 
Further exhumation of these rocks introduces the rocks close to the surface, where 
deformation changed from the ductile deformation to a brittle deformation. Here 
temperature becomes very low and hence deformation at this stage was mainly brittle. This 
is evident in the rocks with various faults, fractures and joints observed in the rocks at 
macroscale. The D3 deformational event is responsible for the crushing and producing the 
fault breccia. The D3 deformations postdate both the D1 and D2 deformations. This stage of 
deformations is characterized by sinistral sense of shear as observed in the fractured thrusted 
pyroxenite in the mafic garnet gneisses. 
The pyroxene bearing amphibolite has a gradational contact with the mafic garnet gneisses. 
This suggests that the pyroxene bearing amphibolite may have metamorphosed into the 
mafic garnet gneisses during prograde metamorphism thus from amphibolite facies to 
granulite facies. The pyroxenite rocks are older materials thrusted within the mafic garnet 
gneisses. The fault breccia formed as a result of crushing and cataclasis of the mafic garnet 
gneisses during brittle deformation. 
Geochemical data show that the mafic garnet gneisses have major elements and trace 
elements pattern concentration similar those of the Lower Crust. The high pressure mafic 
garnet gneisses and associated rocks despite evidences of major and trace elements 
depletions associated with high temperature metamorphism, imprints of the trace elements 
pattern suggest magmatic protolith and with inference that the suture zone mafic granulites 
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consist of rocks largely of Island arc tholeiites (IAT) but also include suites of rocks with 
oceanic crust (MORB) geochemical signatures. This association of rock suites suggests the 
existence of a subduction zone complex where oceanic crust may have formed in either a 
back arc basin or intra-arc basin environment. 
From the Rare Earth Element (REE) and incompatible trace elements plots, the mafic garnet 
gneisses have similar pattern to the lower crust (LC). The rocks generally displayed slight 
enrichments in the Light Rare Earth Elements (LREEs) relative to the Heavy Rare Earth 
Elements (HREEs) with small positive Europium anomaly. They also displayed enrichment 
in the LILE comapared to HFSE. The Magnessium number (Mg#) obtained for the mafic 
garnet gneisses is also approximately equal to that of the Lower Crust (LC). The mafic 
garnet gneisses also displayed almost the same pattern as IAT from the incompatible trace 
elements plots normalized to N MORB. Geotectonic plots also showed that, the rocks have 
affinity for Island Arc Tholeiite (IAT) mainly with few MORB like materials. The rocks 
plotted mainly in the field of Island Arc Tholeiite (IAT), with few plots in the MORB and 
OIA. These results agree with the findings of Attoh and Morgan (2004). 
The associations of the Island Arc Tholeiite (IAT) and the MORB imprints suggest that 
there may have been the formation of oceanic crustal materials within the West African 
Craton (WAC), in either a back arc basin or intra-arc basin environment. These materials 
together with part of the WAC later may have subducted to lower crustal or to mantle depth 
before exhumation by probable uplift or erosions.  
 
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6.2   Recommendation 
It is recommended that detailed structural and petrologcal mapping need to be extended 
throughout the suture zone in Ghana in future research to permit better understanding of the 
suture zone as far as structures and petrology are concerned. 
Also important geochemical techniques such as microprobe for mineral chemistry, 
geochronology and isopotic studies should be carried out together with 
geothermobarometric calculations on the mafic garnet gneisses and its associated rocks that 
occur in the suture zone to provide complementary and useful data for thermobarometry and 
age of the HP metamorphism deductions. 
 
 
 
 
 
 
 
 
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APPENDIX 
  Table 1: Some selected data 
                  Pyroxene Garnet Gneiss                  
  S11        S15        S16A       S20      S24      N58A      N58h    N58f     N62A       N63         N67       N68 
Zr/Hf 38.46154 32.5 30 30.90909 36.875 31.25 33 36.66667  35.55556 32.30769 42.5 40.90909 
Th/U 1.9 1 1 3.6 1.136364 1 2 1  1.5 2.222222 1 1.4 
K/U 46487.28 61429.62 43166.76 81352.74 19243.92 48147.54 58109.1 53128.32  52920.79 48885.43 49807.8 87993.78 
Rb/Cs 80 56.66667 60 75 80 25 100 50  190 230 50 70 
Sr/Nd 58.11475 49.79167 83.93258 67.89474 47.11111 74.78261 71.47059 85.82418  101.6418 67.26316 427.1429 52.44898 
La/Nb 3.36 3.076923 1.666667 1.314815 2.486486 1.5 3.466667 1.722222  5.333333 3.25 0.28 3.947368 
Sm/Nd 0.202459 0.225694 0.280899 0.227193 0.222963 0.283696 0.228431 0.278022  0.244776 0.250526 0.22 0.220408 
U/Pb 0.01 0.025 0.025 0.025 0.11 0.025 0.025 0.025  0.04 0.045 0.025 0.025 
Eu/Eu* 1.361682 1.16659 1.14548 1.166483 1.158913 1.20914 1.278203 1.272986  1.547789 1.234954 1.536499 1.209778 
LaN/YbN 5.899194 5.186104 2.268921 5.092313 5.851491 1.576037 4.552995 3.119403  4.433053 4.979839 28.31613 5.056452 
LaN/SmN 2.139219 1.548387 0.880645 1.724374 1.922624 0.867631 1.403849 0.770751  1.84107 1.717945 1.715543 1.456093 
CeN/YbN 5.065491 4.87481 2.313047 4.595402 5.026854 1.64604 4.299859 3.435976  3.578767 4.173886 24.31436 4.837005 
CeN/SmN 1.836894 1.455446 0.897772 1.556109 1.651673 0.906168 1.325798 0.848971  1.48628 1.439908 1.473094 1.3929 
EuN/YbN 3.25822 3.390372 2.734171 2.994789 3.165447 1.846453 3.471331 4.159204  3.194111 2.972789 19.90476 3.582857 
Zr/Nd 4.098361 2.708333 2.696629 2.982456 4.37037 2.717391 3.235294 3.626374  4.776119 4.421053 4.857143 3.061224 
K/Rb 1452.728 1806.754 3597.23 677.9395 1323.02 2407.377 2905.455 2656.416  2228.244 1912.908 2490.39 3142.635 
Rb/Cs 0.004513 0.002371 0.000803 0.007752 0.005031 0.001453 0.001372 0.00128  0.00279 0.003599 0.000669 0.001816 
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Table 2: Some Selected Data                           
  Garnet Hornblende Gneiss           
Pyroxene Bearing 
Amphibolite Fault Breccia Pyroxenite 
  M2 M5A M5B M3 M4A M4B M6C   M5C M5D   N58d   N58g   
Zr/Hf 40.35 30 33.33 42 40 36.92 30   31.66 28.42   31.66   32.94118   
Th/U 1.12 1 2 2.090909 1.285714 2 1.8   1.8 0.833333   1   3   
K/U 34242.86 1791.34 1791.34 814.2455 2559.057 1279.529 895.67   97955.34 80245.9   6641.04   24903.9   
Rb/Cs 60 130 100 40 190 70 60   150 100   8.333333   38   
Sr/Nd 61.60305 78.36957 72.16981 23.1746 42.89773 62.57143 57.22628   9.62766 9.893617   303.5714   16.07143   
La/Nb 0.919355 2 1.736842 2.06 4.692308 1.428571 2.65   0.833333 0.53   0.513514   2.111111   
Sm/Nd 0.273282 0.272826 0.293396 0.228571 0.2625 0.255 0.278832   0.254255 0.276596   0.25   0.2625   
U/Pb 0.04 0.025 0.025 0.055 0.035 0.035 0.05   0.025 0.03   0.005556   0.035   
Eu/Eu* 0.913034 1.078565 1.014354 0.836539 0.923315 0.982159 0.97139   0.910539 0.978195   1.43724   0.702345   
LaN/YbN 2.066077 1.599781 1.794225 3.306759 3.639452 3.111663 3.080367   4.262143 3.435794   2.099947   1.515938   
LaN/SmN 1.001532 0.701709 0.667462 1.124832 0.83054 1.057197 0.872741   0.723782 0.641129   1.138249   0.813035   
CeN/YbN 2.099902 1.709809 1.981695 3.559701 4.120301 3.183549 3.188695   5.351656 4.402252   2.035384   1.729524   
CeN/SmN 1.017928 0.74997 0.737202 1.210873 0.940273 1.081621 0.903433   0.908799 0.821473   1.103253   0.927586   
EuN/YbN 1.727525 2.216995 2.384902 2.139423 3.246162 2.581057 2.843537   4.608492 4.374673   2.703691   1.245099   
Zr/Nd 8.625954 2.608696 3.773585 5.833333 3.409091 3.428571 2.408759   3.031915 2.87234   4.52381   5   
K/Rb 4565.715 68.89769 89.567 111.9588 94.28105 127.9529 74.63917   3265.178 4814.754   664.104   917.5121   
Rb/Cs 0.000743 0.001803 0.001307 0.00137 0.002517 0.000799 0.001531   0.008287 0.005376   0.000392   0.010556   
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Table 3: Some trace element ratios of the mafic garnet gneiss. 
Sample     La/Nb K/Nb Zr/Hf    Nb/Ta Zr/Nb Ba/Nb     Ce/Pb 
S11 3.36 1859.49 38.4615 25 20 102.40 1.88 
S15 3.07 1181.33 32.5 13 15 65.38 9.80 
S16A 1.66 1027.78 30 21 11.42 43.95 4.65 
S20 1.31 753.26 30.90 27 6.29 35.92 8.35 
S24 2.48 1144.23 36.87 18.5 15.94 61.35 10.30 
N58A 1.50 1003.07 31.25 24 10.41 31.75 4.90 
N58h 3.46 1936.97 33 15 22 74 6.40 
N58f 1.72 1475.78 36.66 18 18.33 48.77 4.45 
N62A 5.33 4704.07 35.55 9 35.55 218.88 5.05 
N63 3.25 2199.84 32.30 20 21 94 7.1 
N67 0.28 332.05 42.5 18.75 2.26 14.13 2.35 
N68 3.94 2315.62 40.90 19 23.68 111.05 9.35 
M2 0.91 441.84 40.35 15.50 18.22 18.30 7.55 
M5A 2 63.97 30 14 17.14 64.64 3.90 
M5B 1.73 47.14 33.33 19 21.05 53.42 4.75 
M3 2.06 17.91 42 25 29.40 13.22 14.45 
M4A 4.69 137.79 40 13 46.15 110.76 9 
M4B 1.42 21.32 36.92 14 11.42 27.50 8 
M6C 2.65 44.78 30 20 16.50 53 7.15 
 
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