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. University of Ghana http://ugspace.ug.edu.gh ii 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) University of Ghana http://ugspace.ug.edu.gh iii 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. University of Ghana http://ugspace.ug.edu.gh iv 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. University of Ghana http://ugspace.ug.edu.gh v DEDICATION I dedicate this work to the Holy Trinity, my late father Mr. Philip Kojo Fosu and to my ever supporting and loving family. University of Ghana http://ugspace.ug.edu.gh vi 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. University of Ghana http://ugspace.ug.edu.gh vii 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 University of Ghana http://ugspace.ug.edu.gh viii 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 University of Ghana http://ugspace.ug.edu.gh ix 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 University of Ghana http://ugspace.ug.edu.gh x 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 University of Ghana http://ugspace.ug.edu.gh xi 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 University of Ghana http://ugspace.ug.edu.gh xii 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), University of Ghana http://ugspace.ug.edu.gh xiii 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 University of Ghana http://ugspace.ug.edu.gh xiv 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 University of Ghana http://ugspace.ug.edu.gh xv 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 University of Ghana http://ugspace.ug.edu.gh xvi 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 University of Ghana http://ugspace.ug.edu.gh xvii 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 University of Ghana http://ugspace.ug.edu.gh xviii 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 University of Ghana http://ugspace.ug.edu.gh xix Px – Pyroxene Cpx – Clinopyroxene Opx – Orthopyroxene Hbl – Hornblende Qtz – Quartz Chl – Chlorite Op – Opaque mineral University of Ghana http://ugspace.ug.edu.gh 1 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 University of Ghana http://ugspace.ug.edu.gh 2 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. University of Ghana http://ugspace.ug.edu.gh 3 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. University of Ghana http://ugspace.ug.edu.gh 4 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)) University of Ghana http://ugspace.ug.edu.gh 5 Figure 1.2: Topographical Map of the study area. University of Ghana http://ugspace.ug.edu.gh 6 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). University of Ghana http://ugspace.ug.edu.gh 7 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 University of Ghana http://ugspace.ug.edu.gh 8 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 University of Ghana http://ugspace.ug.edu.gh 9 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. University of Ghana http://ugspace.ug.edu.gh 10 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 University of Ghana http://ugspace.ug.edu.gh 11 (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. University of Ghana http://ugspace.ug.edu.gh 12 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). University of Ghana http://ugspace.ug.edu.gh 13 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. University of Ghana http://ugspace.ug.edu.gh 14 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). University of Ghana http://ugspace.ug.edu.gh 15 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. University of Ghana http://ugspace.ug.edu.gh 16 Figure 2.1: Tectonic map of the Dahomeyides of southeastern Ghana (after Attoh, 1998) showing the location of the study area. University of Ghana http://ugspace.ug.edu.gh 17 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). University of Ghana http://ugspace.ug.edu.gh 18 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). University of Ghana http://ugspace.ug.edu.gh 19 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). University of Ghana http://ugspace.ug.edu.gh 20 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. University of Ghana http://ugspace.ug.edu.gh 21 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 University of Ghana http://ugspace.ug.edu.gh 22 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 University of Ghana http://ugspace.ug.edu.gh 23 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 University of Ghana http://ugspace.ug.edu.gh 24 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. University of Ghana http://ugspace.ug.edu.gh 25 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. University of Ghana http://ugspace.ug.edu.gh 26 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%. University of Ghana http://ugspace.ug.edu.gh 27 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. University of Ghana http://ugspace.ug.edu.gh 28 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. University of Ghana http://ugspace.ug.edu.gh 29 Figure 4.1: Outcrop map of the study Area. University of Ghana http://ugspace.ug.edu.gh 30 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. University of Ghana http://ugspace.ug.edu.gh 31 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. University of Ghana http://ugspace.ug.edu.gh 32 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 University of Ghana http://ugspace.ug.edu.gh 33 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). University of Ghana http://ugspace.ug.edu.gh 34 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. University of Ghana http://ugspace.ug.edu.gh 35 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. University of Ghana http://ugspace.ug.edu.gh 36 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 University of Ghana http://ugspace.ug.edu.gh 37 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. University of Ghana http://ugspace.ug.edu.gh 38 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). University of Ghana http://ugspace.ug.edu.gh 39 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. University of Ghana http://ugspace.ug.edu.gh 40 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). University of Ghana http://ugspace.ug.edu.gh 41 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. University of Ghana http://ugspace.ug.edu.gh 42 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. University of Ghana http://ugspace.ug.edu.gh 43 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. University of Ghana http://ugspace.ug.edu.gh 44 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. University of Ghana http://ugspace.ug.edu.gh 45 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 University of Ghana http://ugspace.ug.edu.gh 46 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. University of Ghana http://ugspace.ug.edu.gh 47 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. University of Ghana http://ugspace.ug.edu.gh 48 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. University of Ghana http://ugspace.ug.edu.gh 49 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. University of Ghana http://ugspace.ug.edu.gh 50 Figure 4.14: Photomicrographs of pyroxene bearing amphibolite. (Ppl). Same Abbreviations are used. University of Ghana http://ugspace.ug.edu.gh 51 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. University of Ghana http://ugspace.ug.edu.gh 52 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. University of Ghana http://ugspace.ug.edu.gh 53 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. University of Ghana http://ugspace.ug.edu.gh 54 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. University of Ghana http://ugspace.ug.edu.gh 55 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 University of Ghana http://ugspace.ug.edu.gh 56 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 University of Ghana http://ugspace.ug.edu.gh 57 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. University of Ghana http://ugspace.ug.edu.gh 58 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. University of Ghana http://ugspace.ug.edu.gh 59 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. University of Ghana http://ugspace.ug.edu.gh 60 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. University of Ghana http://ugspace.ug.edu.gh 61 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. University of Ghana http://ugspace.ug.edu.gh 62 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) University of Ghana http://ugspace.ug.edu.gh 63 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. University of Ghana http://ugspace.ug.edu.gh 64 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 University of Ghana http://ugspace.ug.edu.gh 65 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. University of Ghana http://ugspace.ug.edu.gh 66 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. University of Ghana http://ugspace.ug.edu.gh 67 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 University of Ghana http://ugspace.ug.edu.gh 68 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. University of Ghana http://ugspace.ug.edu.gh 69 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. University of Ghana http://ugspace.ug.edu.gh 70 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). University of Ghana http://ugspace.ug.edu.gh 71 Figure 4.30: Structural map of the study area. University of Ghana http://ugspace.ug.edu.gh 72 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