Received: 9 September 2016 Revised: 17 February 2017 Accepted: 21 February 2017DOI: 10.1002/gj.2923R E S E A R CH AR T I C L EGeochemical and Sr‐Nd isotopic records of Paleoproterozoic
metavolcanics and mafic intrusive rocks from the West African
Craton: Evidence for petrogenesis and tectonic setting
Patrick Asamoah Sakyi1 | Solomon Anum1,2 | Ben‐Xun Su3 | Prosper M. Nude1 |
Ben‐Can Su4 | Daniel Kwadwo Asiedu1 | Frank Nyame1 | Daniel Kwayisi1 |1Department of Earth Science, School of
Physical and Mathematical Sciences,
University of Ghana, Accra, Ghana
2Geological Survey Department, Accra, Ghana
3State Key Laboratory of Lithospheric
Evolution, Institute of Geology and
Geophysics, Chinese Academy of Sciences,
Beijing, China
4Changqing Oil Field Company Oil Production
Plant No. 8, Xi'an, China
Correspondence
Patrick Asamoah Sakyi, Department of Earth
Science, School of Physical and Mathematical
Sciences, University of Ghana, P. O. Box LG 58,
Legon, Accra, Ghana.
Email: pasakyi@ug.edu.gh
Funding Information
National Natural Science Foundation of China,
Grant/Award Number: 41522203; Youth
Innovation Promotion Association, Chinese
Academy of Sciences, Grant/Award Number:
2016067
Handling editor: M. SantoshGeological Journal. 2018;53:725–741.Metavolcanics andmafic intrusive rocks of the Paleoproterozoic Birimian terrane in the southeastern
part of theWest African Craton, Ghana, were analyzed for major and trace elements andNd and Sr
isotopic data to constrain the geodynamic evolution of the Birimian Supergroup. Themetavolcanic
rocks consist of metabasalts, meta‐andesites, and amphibolites, whereas the mafic intrusions are
mainly gabbros, hornblendites, and dolerites. The rocks are tholeiitic in composition and show
the classic features of arc magmatism. The metavolcanics display significant enrichments in large
ion lithophile elements (LILE) and light rare earth elements, relative to high field strength elements
(HFSE) and heavy rare earth elements. The multielement patterns of the rocks also show positive
Pb, Ba, Th, and Sr and negative Nb, Ta, and Ce anomalies that are typical characteristics of
subduction‐related magmas. They also have La/Nb ratios <3 and La/Ta ratios <43 that are similar
to other Archean and Birimian greenstone belts in West Africa. The rocks have εNd (2.1 Ga) values
of −0.96 to +2.60, and Ndmodel ages of 2.24–2.51 Ga (TDM1) and 2.16–2.45 Ga (TDM2), indicating
their juvenile character with possible contributions from pre‐Birimian crustal materials in their
sources. The εNd values suggest a depleted source and further indicate that they were probably
produced in an almost entirely oceanic environment with minor influence from the continental
crust. The Nd isotopic results are consistent with the island arc model, which views
Paleoproterozoic terranes of the West African Craton in the context of subduction–accretion
processes. Accordingly, these processes may have played a role in the formation of the Columbia
supercontinent during the Paleoproterozoic (2.1–1.8 Ga) orogenic events.
KEYWORDS
Birimian metavolcanics, island arc, Sr–Nd isotopes, subduction, tectonic setting, West African
Craton1 | INTRODUCTION
The Paleoproterozoic era (2.5–1.6 Ga) represents the main episode of
crustal growth recorded on present‐day continents (Giustina et al.,
2009). The Paleoproterozoic was marked by a series of geotectonic
events leading to the formation of orogenic belts and was character-
ized by large‐scale collisional and postcollisional magmatic activity
evidenced in most of the ancient cratons. This global event is related
to the assembly of the supercontinent Columbia in the Late
Paleoproterozoic–Mesoproterozoic (Meert, 2012; Rogers & Santosh,
2002; Zhao, Sun, Wilde, & Li, 2004), notably at about 1.90–1.85 Ga
(Rogers & Santosh, 2009). Zhao, Cawood, Wilde, and Sun (2002), onwileyonlinelibrary.com/jouthe other hand, indicated that the Columbia supercontinent built up
during Paleoproterozoic (2.1–1.8 Ga) orogenic events. The superconti-
nent Columbia is considered the first coherent supercontinental
assembly on Earth (Meert, 2014; Nance, Murphy, & Santosh, 2014;
Rogers & Santosh, 2002; Santosh, 2010). Rogers and Santosh (2002)
proposed that between 1.9 and 1.5 Ga, the eastern part of India was
once connected to the western part of North America, whilst South
America was sutured onto West Africa as part of Columbia.
TheWest African Craton (WAC; Figure 1), affected by the Eburnian
orogeny at around 2 Ga, is composed of three Archaean and
Paleoproterozoic metamorphic and magmatic shields separated by
two cratonic sedimentary basins. The WAC components include thernal/gj Copyright © 2017 John Wiley & Sons, Ltd. 725
726 SAKYI ET AL.
FIGURE 1 Simplified geological map of the Man Shield of the West African Craton (WAC) and Paleoproterozoic Birimian rocks, showing the
location of the study area (modified after Feybesse et al., 2006) [Colour figure can be viewed at wileyonlinelibrary.com]Man shield to the south, the Reguibat shield to the north, and the Anti‐
Atlas belt to the extreme north. In between are the hugeTaoudeni Basin
in the centre and theTindouf Basin to the north. TheMan and Reguibat
shields comprise Archaean nuclei to the west (Feybesse &Milési, 1994;
Potrel, Peucat, & Fanning, 1998). In the Man shield, a large part of the
WAC consists of the Paleoproterozoic Birimian rocks (Abouchami,
Boher, Michard, & Albarède, 1990; Boher, Abouchami, Michard,
Albaréde, & Arndt, 1992; Liégois, Claessens, Camara, & Klerkx, 1991;
Figure 1). The Reguibat, on the other hand, contains Paleoproterozoic
assemblages in the eastern part as well as Archaean components.
For nearly a century, the Paleoproterozoic Birimian rocks of Ghana,
especially the Ashanti Greenstone Belt, have received much attention,
mainly because of their economic potential. These rocks hostmanymin-
eral resources including gold, diamond, manganese, and bauxite, among
others (Kesse, 1985). The Paleoproterozoic Birimian rocks comprise six
greenstone belts with intervening sedimentary basins, namely, Kibi‐
Winneba Belt, Cape Coast Basin, Ashanti Belt, Kumasi Basin, Sefwi Belt,
Sunyani Basin, Bui Belt, Maluwe Basin, Bole‐Nangodi Belt, and Lawra
Belt (Davis, Hirdes, Schaltegger, & Nunoo, 1994; Hirdes & Davis,
1998; Hirdes, Davis, & Eisenlohr, 1992; Hirdes, Davis, Lüdtke, & Konan,
1996; Loh & Hirdes, 1996, 1999). Of the above‐mentioned belts, the
Kibi‐Winneba Belt is the one in contact with the Pan‐African Orogenic
Belt to the east (Figure 1).
The genetic processes of the Paleoproterozoic Birimian rocks of
the WAC have long been the subject of debate. One school of thought
is of the view that the Paleoproterozoic Birimian rocks were generatedfrom plume‐relatedmagmatism (e.g., Abouchami et al., 1990; Boher et al.,
1992; Pouclet, Vidal, Delor, Simeon, & Alric, 1996), but another has pro-
posed that the entire Birimian rocks formed in an island arc environment
(Mortimer, 1992; Sylvester & Attoh, 1992; Ama Salah, Liégeois, &
Pouclet, 1996; Dia, Van Schmus, & Kröner, 1997; Béziat et al., 2000;
Asiedu et al., 2004; Dampare, Shibata, Asiedu, & Osae, 2005; Attoh,
Evans, & Bickford, 2006; Dampare, Shibata, Asiedu, Osae, & Banoeng‐
Yakubo, 2008). In Ghana, studies carried out on the Paleoproterozoic
Birimian rocks have concentrated mostly on the Ashanti and the Sefwi
Greenstone Belts, whereas the Kibi‐Winneba Greenstone Belt, occupy-
ing the southeastern part of theWAC, has received little attention.More-
over, the authors of this study have not come across literature on Sr‐Nd
isotope studies on rock samples in the Kibi‐Winneba Greenstone Belt of
Ghana. Therefore, as a contribution to the ongoing debate about the
geodynamic evolution of the Paleoproterozoic Birimian rocks, this paper
presents geochemical and first ever Sr‐Nd isotopic data for the
metavolcanics and associated mafic intrusions of the Kibi‐Winneba
Greenstone Belt, to provide better understanding of the evolutionary
history of the Paleoproterozoic Birimian Supergroup of the WAC.2 | GEOLOGICAL SETTING
The Paleoproterozoic Birimian rocks and associated syn‐ to late‐
kinematic intrusions mark a major juvenile crust‐forming event referred
to as the “Eburnean orogeny” (Dabo & Aïfa, 2011). Geochronological
SAKYI ET AL. 727studies indicate ages ranging from 2.27 to 2.05 Ga (Taylor, Moorbath,
Leube, & Hirdes, 1988, 1992; Abouchami et al., 1990; Liégois et al.,
1991; Boher et al., 1992; Hirdes et al., 1992, 1996; Davis et al.,
1994; Ama Salah et al., 1996; Bossière, Bonkoungou, Peucat, & Pupin,
1996; Kouamelan, 1996; Kouamelan, Peucat, & Delor, 1997; Doumbia
et al., 1998; Hirdes & Davis, 1998; Loh & Hirdes, 1999; Sakyi et al.,
2014; Anum et al., 2015). In southern and northeastern Ghana and
southeast Côte d'Ivoire, the basin and belts of the PaleoproterozoicFIGURE 2 Geological sketch map of the Kibi‐Asamankese area showing the
[Colour figure can be viewed at wileyonlinelibrary.com]Birimian terrane are NE–SW trending (Figure 2). and those in the
northern part of Côte d'Ivoire, southwest Burkina Faso, and northwest
Ghana trend nearly N–S. The basins are bounded on the east and west
by major strike‐slip fault zones. The volcanic belts constitute mainly
low‐grade metamorphosed lavas of predominantly tholeiitic composi-
tion, minor amphibolites, as well as “Belt type” tonalite‐granodiorite
intrusions. Zircon U–Pb geochronology of some of the granitic plutons
yielded ages of 2179 Ma (Sefwi Belt) and 2172 Ma (Ashanti Belt;distribution of the various rock units (modified after Anum et al., 2015)
728 SAKYI ET AL.Hirdes et al., 1992). Sakyi et al. (2014) have reported new zircon U–Pb
ages of 2213–2211 Ma for granitoids in the Lawra Belt. The belt gran-
itoids are interpreted to be coeval with volcanic rocks of the Birimian
Supergroup.
The Kibi‐Winneba Greenstone Belt forms part of the
Paleoproterozoic Birimian domain of the WAC and trends NE–SW.
The Kibi segment of the belt forms the Atiwa mountain range, which
extends for more than 70 km. This greenstone belt is characterized by
volcanic lobes, made up of basaltic flows, andesitic lavas, pyroclastic
and sedimentary rocks, with different types of granitoids, often with
porphyritic textures occupying intervening positions. Mafic‐ultramafic
plutonic rocks also occur within both the granitoids and the volcanic
rocks. Abouchami et al. (1990) discovered that Birimian rocks in the
study area are generally metamorphosed up to greenschist facies,
although various degrees of contact metamorphism have been
recorded in the rocks close to the granite batholiths. The rocks have also
been affected by low‐grademetasomatic alterations, involving silicifica-
tion and widespread formation of secondary chlorite and sericite.3 | METHODOLOGY
Thirty representative samples of metavolcanic and mafic rocks were
taken from the field. They were analyzed for whole‐rock major and
trace element compositions and also Sm‐Nd and Rb‐Sr isotopic
compositions at the Institute of Geology and Geophysics, Chinese
Academy of Sciences, Beijing.
Major element compositions were obtained using Shimadzu XRF‐
1500 instrument on fused glass disks. Analytical uncertainties were
~1–3% relative for elements present in concentrations >1 wt.% and
~10% relative for elements present in concentrations <1 wt.%. Trace
element concentrations were determined by inductively coupled
plasma mass spectrometry using an Agilent 7500a system. Samples
were digested using a mixture of ultrapure HF and HNO3 in Teflon
bombs. Peridotite reference materials JP‐1 andWPR‐1 were measured
to monitor the accuracy of the analytical procedure, and the results
were in good agreement with reference values. Precisions of the
inductively coupled plasma mass spectrometry were generally better
than 5%. Detail description of analytical procedures is reported in
Chu et al. (2009).
Rb‐Sr and Sm‐Nd isotopes were determined using a VG‐354
Thermal Ionization Magnetic Sector Mass Spectrometer. Procedures
of chemical separation and isotopic analyses were after Zhang et al.
(2001). About 100 mg of sample powders were weighed into 7 ml
Savillex™ Teflon beakers and spiked with appropriate amounts of
87Rb‐84Sr and 149Sm‐150Nd spikes. The samples were dissolved using
a mixed acid of 2 ml HF and 0.2 ml HClO4 on a hotplate at 120 °C
for more than 1 week. After the samples were completely dissolved,
the solutions were dried down on a hotplate at 120 °C and then heated
to 180 °C to completely remove HF. The sample residues were
redissolved in 4 ml of 6M HCl at 100 °C overnight and then dried
down again. Finally, the samples were completely dissolved in 1 ml of
2.5M HCl solution. The Rb, Sr, and rare earth elements (REEs) were
separated from the matrix elements using cation exchange columns
packed with 2 ml AG50W × 12 resins (200–400 mesh). Subsequently,the Nd and Sm were separated from other REEs using HEHEHP
(2‐ethylhexyl phosphonic acid mono‐2‐ethyl hexyl ester) chromato-
graphic columns or Eichrom‐LN columns. Total procedural blanks were
<100 pg for Rb, 200 pg for Sr, <20 pg for Sm, and <50 pg for Nd.
Rb and Sm isotopes were measured using single tantalum filament.
Sr and Nd isotopes were determined using single tungsten filament
with TaF5 as an ionization activator. Measured
87Sr/86Sr and 143Nd/
144Nd ratios were corrected for mass fractionation using 86Sr/
88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. During the
period of data collection, the measured values for the NBS987‐Sr
and JNdi‐Nd standards were 87Sr/86Sr = 0.710245 
 20 (n = 10) and
143Nd/144Nd = 0.512125 
 15 (n = 8), respectively. Uncertainties of
Rb/Sr and Sm/Nd ratios were less than 2% and 0.5%, respectively.4 | RESULTS
4.1 | Field relations and petrography
4.1.1 | Metavolcanics
The metavolcanic rocks outcrop mostly in the north‐central part of the
study area (Figure 2) and they consist of metabasalts or basaltic meta‐
andesites and amphibolites. The former are mostly aphyric and
sparsely porphyritic, with less than 10% phenocrysts set in a micro-
crystalline groundmass. They are generally weakly foliated; however,
some varieties are much more foliated and highly chloritized with
steep dips (65°–70° SE). The primary minerals of the metabasalts or
basaltic meta‐andesites are mainly andesine, clinopyroxene, and minor
orthopyroxene. The andesine occurs as subhedral phenocryst and is
partially or completely replaced by sericite, epidote, and calcite, whilst
the pyroxene is partially or completely replaced by chlorite, epidote,
and in some samples, actinolite (Figure 3). Relict pyroxene is domi-
nantly clinopyroxene in most samples. The groundmass consists of
lath‐like microlite plagioclase, secondary quartz, carbonate, chlorite,
and sericite. Apatite, magnetite, and ilmenite occur as the main acces-
sory minerals. The amphibolites occur at the southern part of the Kibi‐
Winneba Belt and are intruded by mafic dykes and pegmatites. The
amphibolites vary from dark grey to green and are commonly charac-
terized by well‐developed foliations, with fine laminations of pelitic
or silty particles of quartz–calcite bands. Based on field observations
and petrographic studies, the fine‐grained nature of the amphibolites
suggests basaltic rather than gabbroic protolith. All varieties of the
metavolcanic rocks have undergone greenschist‐facies metamorphism,
which has overprinted the primary minerals. This is evidenced by the
mineral assemblages of secondary actinolite, epidote, sericite, and
chlorite observed in microscopic view.4.1.2 | Mafic intrusions
The mafic intrusions are exposed along the banks of the Birim River
around Kibi and the Ayensu River between Anum Apapam and
Asamankese. There are two types of mafic intrusions, namely, (a)
mainly gabbros, hornblendites, and diorites (S006, S007, and S008;
Figure 2) that are weakly foliated, metamorphosed, and occur as iso-
lated bodies within the metavolcanics and whose ages are comparable
to that of the metavolcanics (Geological Survey Department, GSD,
SAKYI ET AL. 729
FIGURE 3 Photomicrographs of (a,b) gabbro showing altered plagioclase (plg) to sericite (ser) and pyroxene (px) to chlorite (chl); (c,d) metavolcanic
rocks showing altered plagioclase (plg) to sericite (ser), and altered pyroxene (px) to epidote (ep)/chlorite (chl; crossed polars) [Colour figure can be
viewed at wileyonlinelibrary.com]2009); (b) massive, fresh, linear intrusions, made up of mostly dolerite
(SA47, SA51, SA65, and SA67) that occur as rounded to subrounded
bodies. Recent U–Pb dating of doleritic dyke swarm in southern Ghana
yielded a preliminary age of 870 Ma (Lenka Baratoux et al., 2014). The
dykes are interpreted to have intruded the Birimian rocks during
the Mesozoic, most likely from Late Triassic to Early Jurassic (e.g.,
Geological Survey Department, GSD, 2009; May, 1971). In the field,
the dolerite dykes have intruded all the other rock types, and these
relations strongly indicate that the dykes are relatively younger in age.4.2 | Major and trace elements
The chemical compositions of the Kibi‐Asamankese metavolcanics and
mafic intrusions in the area are presented in Table 1. They are charac-
terized by SiO2 contents of between 42.44 wt.% and 56.15 wt.% with
an average of 50.27 wt.%; Al2O3 varies from 6.25 wt.% to 14.01 wt.%
with an average of 11.85 wt.%, whereas Fe2O3T ranges from 7.5–
17.4 wt.% with a mean value of 14.09 wt.%. CaO contents also range
from 5.64 to 12.09 wt.% and MgO range from 3.13 to 14.21 wt.%.
TiO2 has restricted contents of 0.40–2.52 wt.%. The TiO2 contents
for the metavolcanics (0.47–2.35 wt.%) and mafic intrusions (0.40–
2.52 wt.%; Table 1) are similar to those of igneous rocks from mag-
matic arcs (Pearce & Cann, 1973) and basalts of intra‐plate settings,
with the latter often retaining high TiO2 concentrations (>2.00 wt.%).
The samples have fairly high concentrations of Cr, Co, V, and Ni.
The metavolcanics display a wide range of Cr contents, varying from
13.9 to 441 ppm, except sample SA55, which contains higher concen-
tration of 1295 ppm. The rest are V (137–429 ppm), Co (23.2–
58.1 ppm), and Ni (23.7–457 ppm). Comparatively, the mafic intrusive
rocks have relatively restricted ranges of these elements as follows; Cr(84.9–291 ppm) except sample S007, which contains higher concen-
tration of 2016 ppm, V (286–540 ppm), Co (52.7–59.2 ppm), and Ni
(82.8–276 ppm). The wide range of concentrations for the
metavolcanics, compared with the limited range for the mafic intrusive
rocks could be attributed to post‐emplacement modification of the for-
mer, most likely by metamorphism, thereby changing their composi-
tion. Samples SA55 and S007 contain the highest concentrations of
MgO (14.2 wt.% for each) and the lowest TiO2 (0.84, 0.40 wt.%) and
Na2O (0.59, 1.10 wt.%) contents, respectively. The high MgO and Cr
contents may connote derivation of these rocks from primitive magma
(Wilson, 1989; Winter, 2001).
With the exception of three metavolcanic samples (S040, SA56,
and SA55) and one mafic intrusive sample (S007) that plot within the
calc‐alkaline field, all the samples show tholeiitic affinity on the binary
SiO2 versus FeO/MgO and the AFM diagrams (Figure 4a; Miyashiro,
1974; Figure 4b). On the AFM diagram, the samples define a composi-
tional trend of increasing Fe2O3T concentrations with decreasing MgO
concentrations and relatively constant Na2O + K2O concentrations,
typical of tholeiitic basalts (Sylvester & Attoh, 1992).
The tholeiites have a wide range of MgO contents (2–14 wt.%).
Petrographic studies and field observations show that the
Paleoproterozoic Birimian rocks from the area have experienced hydro-
thermal alteration and low‐ to medium‐grade regional and contact
metamorphism that may have resulted in major element mobility, espe-
cially of SiO2, MgO, CaO, K2O, andNa2O. It is, therefore, appropriate to
examine geochemical plots such as Zr/TiO2 versus Nb/Y (Winchester &
Floyd, 1977) using the immobile and incompatible elements.
On the Zr/TiO2 versus Nb/Y plot of Winchester and Floyd (1977;
Figure 4c), the metavolcanics and mafic intrusions plot almost exclu-
sively in the basalt and basaltic andesite fields, with one sample each
730 SAKYI ET AL.
TABLE 1 Whole‐rock major and trace element compositions of Paleoproterozoic metavolcanic and mafic intrusive rocks from the Kibi‐Asamankese area of the Kibi‐Winneba Volcanic Belt of Ghana
Metavolcanics Mafic intrusives
S005 S009 S011a S034 S035 S037 S040 S044 S048 SA50 S053 SA54 SA55 SA56 SA58 SA60 SA62 S006 S007 S008 SA47 SA51 SA65 SA67
Wt%
SiO2 56.2 51.0 50.4 48.6 51.1 51.7 50.4 54.2 55.8 46.5 51.9 51.6 48.9 54.6 42.4 54.1 49.0 46.3 53.3 49.1 46.3 49.6 46.5 46.9
TiO2 1.00 2.35 0.90 1.85 1.90 1.23 0.63 1.47 1.36 0.78 1.70 1.02 0.84 0.47 0.87 1.72 1.43 2.52 0.40 2.03 2.12 2.45 1.93 1.99
Al2O3 13.7 12.3 12.5 11.2 11.5 13.1 10.4 10.1 10.5 11.4 12.5 12.2 9.9 13.0 12.1 10.8 10.4 13.3 6.25 13.5 13.1 12.8 14.0 13.8
Fe2O3T 10.3 17.2 13.8 17.4 17.2 15.8 11.6 16.7 14.4 11.5 15.6 13.7 12.2 7.5 11.8 16.2 14.8 15.5 10.9 16.6 15.4 15.6 13.4 13.1
MnO 0.29 0.24 0.26 0.25 0.19 0.23 0.20 0.22 0.22 0.19 0.20 0.17 0.24 0.12 0.18 0.19 0.20 0.21 0.22 0.22 0.23 0.22 0.20 0.21
MgO 3.13 3.34 6.63 5.08 4.34 4.92 10.1 4.57 4.02 8.35 3.70 6.14 14.2 6.75 8.00 4.66 5.89 6.57 14.2 4.38 6.89 5.30 7.39 7.32
CaO 7.69 9.88 10.7 5.79 8.46 7.98 11.8 7.60 8.93 8.66 9.45 10.9 7.99 6.45 8.82 9.44 5.64 9.41 12.1 9.15 9.97 9.73 10.6 10.8
Na2O 3.25 2.50 2.70 2.64 2.68 3.89 2.04 2.54 2.78 2.52 3.16 2.56 0.59 3.16 2.45 0.87 2.82 2.37 1.10 2.91 2.00 2.39 2.19 2.26
K2O 2.52 0.29 0.63 0.03 0.41 0.21 0.12 0.14 0.25 0.04 0.36 0.20 1.61 0.22 1.65 0.52 0.34 0.58 0.37 0.79 0.56 0.45 0.84 0.70
P2O5 0.37 0.36 0.09 0.19 0.23 0.13 0.06 0.14 0.13 0.07 0.29 0.14 0.11 0.09 0.09 0.17 0.19 0.40 0.05 0.28 0.28 0.27 0.29 0.28
LOI 1.39 0.48 1.20 6.71 1.83 0.84 2.39 2.19 1.50 9.88 1.03 1.29 3.26 7.13 10.93 1.22 9.11 2.71 1.01 0.81 3.09 1.19 2.68 2.66
Total ppm 99.9 100.0 99.9 99.7 99.9 99.9 99.8 99.8 99.9 99.9 99.9 99.9 99.9 99.5 99.4 99.9 99.9 99.9 99.9 99.8 99.9 99.9 100.0 100.0
V 214 339 308 370 429 414 285 387 419 253 408 340 245 137 312 388 330 403 286 459 540 410 391 380
Cr 190 115 183 39.0 73.0 13.9 406 14.2 18.1 336 35.1 109 1295 441 353 83.2 106 245 2016 111 84.9 156 291 269
Co 23.2 42.2 56.7 38.7 39.2 50.6 52.0 46.1 56.5 37.8 48.0 52.9 58.1 31.4 56.1 47.1 35.5 52.9 53.1 59.2 55.3 53.5 52.7 48.9
Ni 50.9 36.0 94.5 26.5 38.5 23.7 123.0 25.1 36.2 68.0 27.3 110 457 212 119 45.4 57.6 82.8 276 94.9 71.5 82.0 128 119
Cu 8.3 31.7 158 31.3 38.2 17.6 105 118 101 29.5 50.2 74.7 10.2 56.2 123 55.7 39.5 65.0 21.9 306 201 95.7 106 110
Zn 107 137 115 146 130 115 69.0 113 124 71 141 85.1 97.5 60.9 83.1 120.0 85.0 149 61.8 156 155 134 105 101
Rb 69.0 4.77 76.6 0.673 12.0 1.48 4.87 3.23 2.81 1.32 3.31 1.84 35.9 6.78 43.7 16.3 9.19 23.2 11.1 25.6 13.3 12.2 27.8 20.6
Sr 471 232 267 204 266 199 187 98.4 344 151 314 136 79.4 236 232 131 95.1 479 79.9 247 224 253 250 243
Y 30.1 50.1 19.6 37.9 38.1 28.7 17.1 30.3 31.0 18.3 29.8 19.8 14.6 8.0 17.7 28.0 12.7 35.6 8.50 37.2 37.5 29.0 25.4 24.9
Zr 66.2 73.6 20.2 139 121 91.1 37.3 71.2 98.6 50.9 60.3 13.4 65.9 100 56.7 32.3 82.0 221 19.6 211 191 186 142 141
Nb 21.7 12.4 2.81 6.98 7.87 4.07 2.39 4.78 5.01 3.17 9.32 2.72 3.32 3.06 3.31 5.60 4.60 20.0 2.49 10.6 13.4 17.9 12.0 12.4
ppm
Cs 0.97 0.11 4.84 0.13 1.47 0.03 0.11 0.20 0.09 0.35 0.13 0.08 0.63 0.56 0.80 2.32 0.35 2.46 0.83 3.35 0.50 0.69 0.36 0.51
Ba 546 137 155 16.9 149 50.9 86.2 81.3 86.8 30.1 110 38.6 1188 129 338 168 76.1 274 105 376 147 225 210 230
La 19.8 17.6 5.25 12.7 17.4 8.6 6.10 13.7 14.9 5.55 12.0 3.68 6.82 8.61 5.86 12.2 7.06 27.7 6.43 22.3 17.1 20.9 15.6 15.2
Ce 64.4 37.3 11.5 27.9 33.6 19.5 12.9 27.6 30.5 11.4 38.0 8.98 13.9 16.9 13.2 26.6 16.0 53.8 12.6 44.7 36.4 40.4 32.3 32.0
Pr 4.63 5.22 1.65 3.93 4.89 2.70 1.75 3.62 3.87 1.51 4.30 1.36 1.92 2.05 1.91 3.64 2.47 6.77 1.62 5.71 5.00 5.30 4.27 4.23
Nd 19.9 25.7 8.38 20.1 23.4 13.8 8.59 17.4 18.7 7.52 22.1 7.28 9.02 9.30 9.32 17.1 12.3 31.4 7.71 27.0 24.5 24.1 19.6 19.9
Sm 4.39 6.90 2.36 5.37 5.78 3.84 2.31 4.01 4.51 2.16 5.88 2.11 2.30 1.97 2.45 4.28 3.37 7.00 1.71 5.99 6.21 5.49 4.60 4.84
Eu 1.45 2.09 0.94 1.58 1.81 1.25 0.72 1.31 1.52 0.86 2.12 0.83 0.94 0.58 0.83 1.54 1.21 2.39 0.42 1.91 2.20 1.87 1.69 1.53
Gd 4.43 8.22 2.93 6.08 6.37 4.30 2.56 4.57 4.94 2.49 5.73 2.71 2.63 1.71 2.69 4.48 3.15 7.22 1.64 6.73 6.64 5.49 4.75 4.94
Tb 0.93 1.60 0.62 1.22 1.20 0.88 0.53 0.99 0.94 0.50 1.06 0.56 0.47 0.27 0.54 0.88 0.51 1.33 0.27 1.27 1.29 1.00 0.87 0.85
(Continues)
SAKYI ET AL. 731
TABLE 1 (Continued)
Metavolcanics Mafic intrusives
S005 S009 S011a S034 S035 S037 S040 S044 S048 SA50 S053 SA54 SA55 SA56 SA58 SA60 SA62 S006 S007 S008 SA47 SA51 SA65 SA67
Dy 5.78 10.5 4.03 7.62 7.69 5.74 3.24 5.91 5.98 3.24 6.29 3.70 2.87 1.52 3.51 5.54 2.66 7.06 1.65 7.50 7.88 5.88 4.86 4.99
Ho 1.08 1.92 0.77 1.38 1.39 1.07 0.67 1.19 1.13 0.65 1.13 0.71 0.53 0.31 0.65 1.00 0.46 1.28 0.32 1.39 1.35 1.05 0.93 0.89
Er 3.67 5.87 2.41 4.47 4.33 3.21 1.91 3.31 3.41 2.03 3.45 2.24 1.68 0.88 1.98 3.16 1.44 3.73 1.00 4.18 4.32 3.21 2.73 2.55
Tm 0.57 0.93 0.35 0.70 0.65 0.51 0.30 0.53 0.51 0.32 0.49 0.32 0.24 0.13 0.27 0.47 0.22 0.57 0.15 0.62 0.57 0.45 0.39 0.40
Yb 3.39 5.60 2.30 4.30 4.18 3.21 1.85 3.01 3.25 2.09 2.98 2.24 1.45 0.98 1.91 3.10 1.53 3.57 1.00 3.96 3.85 2.82 2.43 2.46
Lu 0.50 0.83 0.33 0.59 0.61 0.51 0.27 0.44 0.48 0.31 0.44 0.33 0.23 0.14 0.25 0.45 0.26 0.53 0.14 0.57 0.60 0.43 0.38 0.36
Ta 1.62 0.83 0.44 0.47 0.56 0.26 0.17 0.32 0.35 0.20 0.58 0.19 0.22 0.23 0.22 0.37 0.32 1.42 0.19 0.69 0.98 1.18 0.84 0.79
Pb 9.71 3.80 7.12 4.87 4.33 3.81 2.33 4.40 6.86 3.31 4.87 1.91 3.01 6.97 4.14 3.60 1.69 5.95 2.66 7.57 7.15 5.67 3.86 3.34
Th 5.16 1.82 0.50 1.67 1.89 1.13 0.72 1.48 1.53 0.63 1.91 0.22 0.78 1.03 0.44 1.12 0.46 2.27 0.61 2.24 2.34 1.92 1.31 1.33
U 0.77 0.48 0.20 0.49 0.51 0.36 0.20 0.43 0.48 0.20 0.44 0.07 0.26 0.45 0.12 0.29 0.11 0.57 0.22 0.47 0.53 0.51 0.32 0.35
(Eu/Eu*) 1.01 0.85 1.09 0.85 0.91 0.94 0.91 0.94 0.98 1.14 1.12 1.06 1.17 0.96 0.99 1.08 1.14 1.03 0.77 0.92 1.05 1.04 1.11 0.96
(Ce/Ce*) 1.62 0.94 0.94 0.95 0.88 0.98 0.95 0.94 0.97 0.95 1.27 0.97 0.92 0.97 0.95 0.96 0.92 0.95 0.94 0.95 0.95 0.92 0.95 0.96
(La/Yb)N 3.94 2.12 1.54 1.99 2.81 1.80 2.22 3.07 3.09 1.79 2.71 1.11 3.17 5.92 2.07 2.65 3.11 5.23 4.34 3.80 2.99 5.00 4.33 4.17
(La/Sm)N 2.84 1.60 1.40 1.49 1.89 1.40 1.66 2.15 2.08 1.62 1.28 1.10 1.87 2.75 1.50 1.79 1.32 2.49 2.37 2.34 1.73 2.39 2.13 1.98
Th/Nb 0.24 0.15 0.18 0.24 0.24 0.28 0.30 0.31 0.31 0.20 0.20 0.08 0.23 0.34 0.13 0.20 0.10 0.11 0.24 0.21 0.17 0.11 0.11 0.11
LOI, loss on ignition.plotting in the fields defined by andesites and subalkaline basalts,
respectively. Accordingly, the rocks can be broadly classified as
subalkaline basalt to andesite. The trend shows a moderate range in
the Nb/Y ratio. On the diagram of Jensen (1976; Figure 4d), all but
seven of the samples are high‐Fe tholeiite basalts, and the remaining
samples are high‐Mg tholeiitic basalts (SA50, SA56, and SA58),
komatiitic basalts (S007, SA55, and S040), and andesite (S005). Also
the samples plot generally in the basalt field except samples SA56
and S005 that plot in the basaltic andesite/andesite and alkali basalt
fields respectively (Pearce, 1996; Figure 5), reflecting the primitive
and high‐MgO nature of the magma.
The trace element compositions of the samples from the area are
presented in Table 1. Chondrite‐normalized REE patterns (Boynton,
1984) for the samples reveal a gradual increase in the concentrations
of the REE, mostly the light rare earth elements (LREE) in the
metavolcanics (Figure 6a), whereas the mafic rocks are differentiated,
with the LREE being enriched relative to the heavy rare earth elements
(HREE; Figure 6b). The metavolcanics and mafic intrusions show weak
negative to weak positive Eu anomalies (Eu/Eu*) of 0.85–1.17 and
0.77–1.11, respectively. Similarly, the metavolcanics display predomi-
nantly weak negative to slightly positive Ce anomalies (Ce/Ce* = 0.88–
1.62), and the mafic intrusions have weak negative Ce anomalies (Ce/
Ce* = 0.92–0.95). The metavolcanics are characterized by flat to HREE
depleted patterns [(La/Sm)N = 1.10–2.84, (La/Yb)N = 1.11–5.92], and
these are identical to those for the mafic intrusions [(La/Sm)N = 1.73–
2.49, (La/Yb)N = 2.99–5.23]. The weak negative and minor positive Eu
anomalies suggest plagioclase fractionation and accumulation, respec-
tively. Related geochemical studies conducted on Paleoproterozoic vol-
canic rocks in Ghana (e.g., Leube, Hirdes, Mauer, & Kesse, 1990; Loh &
Hirdes, 1999; Sylvester & Attoh, 1992) and other Birimian terranes of
the WAC (e.g., Abouchami et al., 1990) have also identified Birimian
tholeiites with similar characteristics, with or without negative Ce
anomalies. On Figure 6a and 6b, the LREE are enriched relative to
the HREE, notably Dy, Ho, Er, Tm, Yb, and Lu. In addition to the
above, some of the metavolcanics and mafic intrusions show
depletion in Y and HREEs relative to Normal Mid‐Ocean Ridge Basalt
(N‐MORB), but the rest are enriched compared to N‐MORB.
Incompatible trace element concentrations of the rocks normal-
ized to the N‐MORB composition (Sun & McDonough, 1989) display
significant enrichments in LILE. The multielement patterns of the rocks
show weak to pronounced positive Pb, Ba, Th, and Sr anomalies and
weak positive to very prominent negative Zr anomalies (Figure 6c
and 6d). All the samples show strong negative Nb and Ta anomalies,
relatively weak Ti anomalies, and weak positive to moderate, and
strong negative Hf anomalies. The geochemical features such as
enrichment in Ba, Sr, and Th and depletion in Nb, Ta, and Ti are typical
characteristics of subduction‐related magmas. Th/Nb ratios for the
metavolcanics range between 0.08 and 0.34, and those for the mafic
intrusive rocks are 0.11–0.24. Two metavolcanic rocks have lower K
values relative to N‐MORB.4.3 | Sr‐Nd isotopes
Eight metavolcanics and two gabbro intrusive samples from the study
area were analyzed for their Sr‐Nd composition and the results are
732 SAKYI ET AL.
FIGURE 4 (a) Binary discrimination diagram of FeOT/MgO versus SiO2, showing calc‐alkaline and tholeiitic trends, (b) AFM diagram for the
Kibi‐Asamankese metavolcanic rocks and mafic intrusions, showing tholeiitic and calc‐alkaline trends, (c) Zr/TiO2 versus Nb/Y classification
diagram for the metavolcanics and mafic intrusive rocks from the Kibi‐Asamankese area (after Winchester & Floyd, 1977), (d) classification of the
Kibi‐Asamankese metavolcanic and mafic intrusive rocks on the cation plot of Jensen (1976) [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 5 Zr/Ti versusNb/Y discriminant diagram (Pearce, 1996) for the
metavolcanics and mafic intrusive rocks from the Kibi‐Asamankese area
[Colour figure can be viewed at wileyonlinelibrary.com]presented in Tables 2 and 3. The initial 87Sr/86Sr ratios and εNd values
were calculated at an age of 2.1 Ga, representing the crustal formation
time during the Eburnean orogeny (e.g., Abouchami et al., 1990; Boheret al., 1992). The metavolcanic rocks have low initial 87Sr/86Sr ratios
(0.69992–0.70262) and low εNd values (i.e., −0.96 to +2.60) at
2.1 Ga. Similarly, the predominantly gabbro intrusive rocks also have
low initial (87Sr/86Sr) ranging from 0.69854 to 0.70054, and low posi-
tive εNd values (+0.46 and +1.50).
Crustal residence ages (TDM) may be calculated employing diverse
models of depleted mantle evolution (Albarède & Brouxel, 1987; Ben
Othman, Polvé, & Allègre, 1984; Blichert‐Toft & Albarède, 1997;
DePaolo, 1981; Goldstein, O'Nions, & Hamilton, 1984; Liew &
Hofmann, 1988; McCulloch, 1987; Su et al., 2011; Zeng, Liang, Peng,
Yu, & Xiang, 2015). In this study, the TDM1 and TDM2 (representing
single‐ and two‐stage Nd model ages, respectively) calculated
according to the depleted mantle model of DePaolo (1981) are pre-
sented in Table 3. The single‐stage model ages (TDM1) are 2.24–
2.51 Ga, whereas the two‐stage Nd model ages (TDM2) are 2.16–
2.45 Ga. Both the two‐stage (TDM2) and single‐stage (TDM1) model
ages are identical, and in both scenarios, the samples recorded
model ages >2.0 Ga, displaying regular patterns with ages older
than the formation age of 2.1 Ga (e.g., Abouchami et al., 1990;
Boher et al., 1992). Similarly, the intrusive rocks have single‐stage
model ages (TDM1) of 2.26–2.41 Ga and two‐stage model ages
(TDM2) of 2.24–2.33 Ga.
SAKYI ET AL. 733
FIGURE 6 Chondrite‐normalized rare earth element spider plots for (a) the Kibi‐Asamankese metavolcanic rocks and (b) mafic intrusive rocks
(Boynton, 1984). N‐MORB‐normalized multielement diagrams for (c) the Kibi‐Asamankese metavolcanic rocks and (d) mafic intrusive rocks (after
Sun & McDonough, 1989). Symbols are the same as in Figure 4a [Colour figure can be viewed at wileyonlinelibrary.com]
TABLE 2 Rb‐Sr isotopic compositions of Paleoproterozoic metavolcanic and mafic intrusive rocks from the Kibi‐Asamankese area of the Kibi‐
Winneba Volcanic Belt, Ghana
Sample # Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr (2s) (87Sr/86Sr)2.1
Metavolcanics
S005 60.0 473 0.3672 0.71087 0.000010 0.69992
S009 4.07 228 0.0516 0.70350 0.000010 0.70196
S011a 67.1 260 0.7465 0.72423 0.000009 0.70197
S034 0.579 204 0.0082 0.70286 0.000008 0.70262
S037 1.24 192 0.0186 0.70277 0.000008 0.70221
S040 4.09 186 0.0635 0.70316 0.000012 0.70127
SA55 32.4 76.9 1.2196 0.73873 0.000015 0.70236
SA56 6.17 235 0.0761 0.70437 0.000011 0.70210
Mafic intrusive
S007 9.76 73.1 0.3862 0.71005 0.000008 0.69854
S008 20.9 225 0.2681 0.70853 0.000011 0.700545 | DISCUSSION
5.1 | Petrogenesis
Petrogenetic studies require comprehensive knowledge of petrogra-
phy, major, minor and trace elements, and isotope geochemistry
(Wilson, 1989). The integration of these can help infer magma
source, degree and condition of generation, differentiation pro-
cesses, and any postemplacement modification that magmas may
have experienced. For example, the appropriate plots of trace ele-
ments can be used to determine the degree of melting, depth of pri-
mary magma generation and identify phases that fractionate from
magma. Accordingly, plots of Ni and Th versus SiO2 (Figure 7) weregenerated, showing a decrease in compatible element concentration
and an increase in incompatible element concentrations with a
steady increase in the SiO2 content. The negative correlation of
SiO2 with Ni (Figure 7) may indicate a possible partitioning of
olivine, spinel, and pyroxene during the evolution of the magma,
suggesting that fractional crystallization may have played an impor-
tant role in the evolution of the rocks, especially the metavolcanics
(Dampare et al., 2008; Tushipokla, 2013). Although partial melting
and crystal fractionation may have contributed significantly in con-
trolling the geochemistry of the various mafic rocks, the remarkably
uniform Nb depletion and enrichment of LILE and LREE suggest the
presence of a dominant mantle source with these characteristics on
a large regional scale.
734 SAKYI ET AL.
TABLE 3 Sm‐Nd isotopic compositions of Paleoproterozoic metavolcanic and mafic intrusive rocks from the Kibi‐Asamankese area of the Kibi‐
Winneba Volcanic Belt, Ghana
ɛNd
Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd (2 s) (2.1 Ga) TDM1 (Ga) TDM2 (Ga) ƒ Sm/Nd
Metavolcanics
S005 3.92 18.0 0.1316 0.511689 0.000011 −0.96 2.51 2.45 −0.331
S009 6.38 24.1 0.1602 0.512233 0.000006 1.96 2.30 2.21 −0.186
S011a 2.19 7.62 0.1735 0.512450 0.000015 2.60 2.24 2.16 −0.118
S034 4.90 18.5 0.1598 0.512246 0.000007 2.32 2.24 2.18 −0.188
S037 3.44 12.4 0.1679 0.512315 0.000008 1.48 2.42 2.25 −0.146
S040 2.28 8.58 0.1607 0.512201 0.000011 1.19 2.42 2.27 −0.183
SA55 2.21 8.94 0.1496 0.512080 0.000008 1.83 2.28 2.22 −0.239
SA56 1.62 8.07 0.1211 0.511643 0.000009 0.99 2.29 2.28 −0.384
Mafic intrusives
S007 1.50 7.10 0.1279 0.511763 0.000016 1.50 2.26 2.24 −0.350
S008 5.52 23.5 0.1422 0.511908 0.000007 0.46 2.41 2.33 −0.277
FIGURE 7 Harker variation diagrams for selected trace elements against SiO2 for the Kibi‐Asamankese metavolcanic rocks and mafic intrusions.
Symbols are the same as in Figure 4a [Colour figure can be viewed at wileyonlinelibrary.com]Major elements such as Si, K, Na, Ca, and some trace elements
(e.g., Cs, Rb, Ba, and Sr) are easily mobilized by late and/or
postmagmatic fluids and also during metamorphism. However, some
studies (Bédard, 1999; Bienvenu, Bougault, Joron, Treuil, & Demitriev,
1990; Holm, 1985; Leybourne, Van Wagoner, & Ayres, 1997;
Shervais, 1982; Tushipokla, 2013) have established that HFSE, REE,
and transition elements (e.g., Ni, Cr, V, and Sc) may be relatively
immobile during alteration and low‐grade metamorphism of basaltic
and more evolved rocks. Accordingly, Figure 8 presents bivariate dia-
grams of selected trace elements versus Zr to evaluate the element
mobility in the Birimian rocks of this part of the WAC. Strong linear
relations between Zr and most of the elements, especially the REE,
transition elements and HFSE, suggest that these elements are rela-
tively immobile.To explain source variations and crustal assimilation, Th/Yb versus
Nb/Yb diagram of Pearce (1983) and Pearce and Peate (1995; Figure 9)
were plotted for the studied rocks, because these ratios are mostly
independent of fractional crystallization and/or partial melting. The
metavolcanics display a broad range of Th/Yb ratios, extending to
lower and higher values, compared to a restricted range for the mafic
intrusions. Additionally, the metavolcanics appear to align parallel to
the MORB array, with only two samples (SA54 and SA62) plotting on
it. Similarly, three mafic intrusive samples (SA67, S006, and SA51) plot
almost on the Mid‐Ocean Ridge Basalt (MORB) array (Figure 9). This
suggests that the parental magma for the five samples were generated
from a mantle source, which has not experienced any contribution
from melted and recycled crustal material. However, theTh/Nb values
of the metavolcanics (0.08–0.34) and mafic intrusions (0.11–0.24) are
SAKYI ET AL. 735
FIGURE 8 Bivariate diagrams for selected trace elements versus Zr abundances of the Kibi‐Asamankese rocks (concentrations are in ppm).
Symbols are the same as in Figure 4a [Colour figure can be viewed at wileyonlinelibrary.com]identical to the values for Enriched Mid‐Ocean Ridge Basalt (E‐MORB)
(0.06–0.08), OIB (0.078–0.157), and continental crust (0.44; Table 6.7;
Rollinson, 1993). This may suggest that these materials may have
interacted with subducted components either at the source or during
ascent. Negative Ce anomaly is generally regarded as an indicator of
crustal contamination (Rollinson, 1993). This could reflect dehydration
of a subducted slab, resulting in the generation of low‐Ce slab‐
derived fluids (Pearce, 1983). According to Neal and Taylor (1989),
negative Ce anomaly can be produced by subduction of pelagic sedi-
ments + seawater‐altered basalt into the mantle or by recycling of
sediment or crustal material into the upper mantle. Therefore, in this
study, the negative anomalies can be inherent in the source region,
confirming the impact that recycled crustal material could have on
the composition of the upper mantle, particularly the recognition of
a crustal signature in the mantle. We, therefore, propose that the sub-
duction process probably introduced crustal material into the source
region, which subsequently remelted together with the mantle
material.
Using a reference isochron at 2.1 Ga, the metavolcanic rocks and
mafic intrusions from the study area provide low initial Sr isotope
(0.69854–0.70262) and negative to low positive epsilon Nd. Previous
studies of different Birimian rocks of the WAC (e.g., Abouchami et al.,1990; Boher et al., 1992; Taylor et al., 1992; Gasquet, Barbey, Adou,
& Paquette, 2003; Dampare et al., 2009; Manu et al., unpublished;
Sakyi et al., unpublished) have produced a wide range of initial 87Sr/
86Sr ratios from as low as 0.653 to 0.706226. Our values are in agree-
ment with those obtained in those studies. The Paleoproterozoic
Birimian rocks of the WAC have been metamorphosed under mostly
greenschist‐facies conditions (Hirdes et al., 1992; Leube et al.,
1990). The low initial 87Sr/86Sr ratios have been attributed to the
alteration of feldspar (e.g., Dampare et al., 2009; Gasquet et al.,
2003). From thin section examination, the plagioclase feldspars were
observed to have either partially or completely altered to epidote,
sericite and calcite (Figure 3). Therefore, we conclude that considering
the age of the Birimian rocks, post‐emplacement modification of the
rocks through alteration and possibly metamorphism may have
affected the initial 87Sr/86Sr ratios. Accordingly, emphasis will not
be entirely placed on the (87Sr/86Sr)i ratios in the interpretation of
our data.
The εNd values of the metavolcanics (−0.96 to +2.60) and mafic
intrusions (+0.46 to +1.50) may suggest mantle‐derived magmas with
variable but limited extent of endogenic contamination (i.e., contami-
nation within the mantle rather than the crust, such as input from
subducted sediments). The juvenile character of the studied rocks is
736 SAKYI ET AL.
FIGURE 9 Th/Yb versus Nb/Yb diagram (after Pearce, 1983; Pearce &
Peate, 1995) of the metavolcanic and mafic rocks illustrating the input
of Th from either subduction‐zone enrichment or crustal
contamination (CC). In this plot, samples with very little subducted slab
influence lie within or very close to the “mantle” array defined by the
N‐MORB, array and samples influenced by subducted slab flux are
displaced from the “mantle” array to higher Th at a given Nb content
than do the former. WPE = within‐plate enrichment; SZ = subduction
zone flux. N‐MORB, E‐MORB, and OIB from Sun and McDonough
(1989). Symbols are the same as in Figure 4a [Colour figure can be
viewed at wileyonlinelibrary.com]established in the plot of εNd (2.1 Ga) versus formation age (t) on which
they plot close to the depleted mantle curve and are also widely sepa-
rated from the Nd isotopic evolutionary trend of Archean crust
(Figure 10). Similar observations were made by Dampare et al. (2009)
in a study carried out on Paleoproterozoic Birimian metavolcanic rocks
from the southern portion of WAC in Ghana. Thus, the trace elementsFIGURE 10 Plot of ɛNd (2.1 Ga) versus formation age (time) for the
Paleoproterozoic metavolcanic rocks and mafic intrusive rocks from
the Kibi‐Asamankese area of the Kibi‐Winneba volcanic belt. Data of
Archean continental crust is from Kouamelan et al. (1997) [Colour
figure can be viewed at wileyonlinelibrary.com]and isotope signatures of the metavolcanic rocks are consistent in sug-
gesting that their parent magma was generated from a depleted mantle
with minor contribution from the crust. This, therefore, corroborates
previous interpretations that the Paleoproterozoic Birimian rocks of
the WAC represent a juvenile crust (Abouchami et al., 1990; Boher
et al., 1992; Hirdes et al., 1992, 1996; Liégois et al., 1991). Leube
et al. (1990) have indicated that most Birimian tholeiites of Ghana pos-
sess N‐MORB chemistry with few showing slight to moderate LREE
enrichment. The coexistence of both flat and LREE‐enriched patterns
of the rocks and εNd values ranging from −0.96 to +2.60 can be
interpreted as indicative of minor or no crustal contamination in the
petrogenesis of the metavolcanics and the gabbro intrusions.
The initial εNd values of the rocks, coupled with geochemical
features such as Nb depletion relative to the LREE, negative Nb‐Ta
anomalies, relatively high LILE/HREE and LILE/HFSE ratios, could
point to subduction‐related lithospheric mantle source. Their high
positive εNd (2.1 Ga) values and Nd model ages are possible indications
of a subduction input of an older crustal material.
The positive εNd values for some of the rocks, including those of the
LREE‐enriched crustal precursors show that their ultimate sources had
been previously depleted in Nd relative to Sm when compared with
average chondritic Sm/Nd ratios. Furthermore, the positive εNd values
of some of the volcanic rocks strongly suggest that they were produced
entirely from the mantle in an oceanic setting (DePaolo, 1988).
Our samples plot almost exclusively within the field defined by
Birimian basalts and granitoids (Figure 11). This, coupled with the
low and restricted (87Sr/86Sr)i values and negative to low positive ini-
tial εNd(T), suggests that rocks from the study area have similarFIGURE 11 ɛNd versus initial 87Sr/86Sr ratios (calculated at 2.1 Ga) plot
for the Paleoproterozoic metavolcanic rocks and mafic intrusive rocks
from the Kibi‐Asamankese area. Also shown is the field of isotopic
composition of Birimian basaltic and felsic rocks (shaded area) from the
West African Craton (adopted from Pawlig et al., 2006 and the
references therein). DM represents contemporaneous depleted mantle
from Ben Othman et al. (1984) [Colour figure can be viewed at
wileyonlinelibrary.com]
SAKYI ET AL. 737isotopic characteristics to rocks from other Birimian terranes in the
WAC (e.g., Abouchami et al., 1990; Boher et al., 1992; Dampare
et al., 2009; Gasquet et al., 2003; Taylor et al., 1992).
The metavolcanic rocks (TDM1 = 2.24–2.51 Ga; TDM2 = 2.16–
2.45 Ga) and mafic intrusions (TDM1 = 2.26–2.41 Ga; TDM2 = 2.24–
2.33 Ga) also display Nd model ages that are slightly different from
the formation age (2.1 Ga). Granitoids intruding into the metavolcanic
rocks in the study area were dated by Anum et al. (2015), yielding an
age bracket of 2193–2127 Ma. These ages preclude any Archean ter-
rane in the study area and therefore could not have contributed
towards the model ages obtained in this study. However, the Nd model
age of ~2600 Ma for the Winneba granitoids (Taylor et al., 1992) sug-
gested that the Birimian in that area is probably underlain by an
Archean basement. Therefore, the model ages obtained in this study
reflect the juvenile character of the rocks with contribution of
reworked pre‐Birimian rocks (or Archean?) in the source material.5.2 | Geotectonic setting
The geochemical data for the metavolcanic and mafic intrusive rocks
were also plotted on the Ti versus V diagram of Shervais (1982), toFIGURE 12 Discriminant diagrams indicating tectonic settings for the stud
setting of the studied rocks, (b) Ti–Mn–P plot (after Mullen, 1983), (c) Nb–
Zr plot of Pearce and Norry (1979). The fields are midoceanic ridge basalt
island‐arc calc‐alkaline basalt (CAB), island arc tholeiites (IAT), and boninite (
within‐plate tholeiites; B = E‐type MORB; C = within‐plate tholeiites and vo
are the same as in Figure 4a [Colour figure can be viewed at wileyonlinelibestablish their tectonic setting. On this, typical island‐arc tholeiites
have Ti or V values ranging between 10 and 20, MORB ranges
between 20 and 50, and oceanic island and alkaline basalts fall
between 50 and 100 (Figure 12a). Calc‐alkaline basalts are plotted
vertically in a field bounded by Ti or V values between 15 and 50.
The volcanic rocks in this study have Ti or V values between 8 and 41
(Figure 12a) and mostly plot in the Mid‐Ocean Ridge Basalt ‐ Volcanic
Arc Basalt (MORB‐VAB) overlap field. On the discrimination diagram
of Mullen (1983; Figure 12b), the studied rocks fall within the
island‐arc tholeiite and N–MORB fields. Ratios of Zr/Y and composi-
tions of Zr for the metavolcanics and mafic intrusive rocks were plot-
ted on the binary discrimination diagram of Pearce and Norry (1979),
where they fall in the field defined by an overlap of the N‐MORB and
island arc (Figure 12c). Similarly, the Nb–Zr–Y discrimination plot of
Meschede (1986; Figure 12d) defines the rocks as within‐plate tholei-
ite with N‐MORB and volcanic arc basalt compositions, because the
Zr/Y values vary at relatively constant Nb concentrations.
In order to further investigate the geotectonic environment in
which the Birimian rocks in the southeastern part of the WAC were
formed, a ƒSm/Nd versus εNd(2.1 Ga) plot (Figure 13) was generated, on
which the samples plot almost exclusively within the field definedied rocks. (a) V versus Ti plot of Shervais (1982) defining the tectonic
Zr–Y discrimination diagram (after Meschede, 1986), (d) Zr/Y versus
(MORB), ocean island tholeiites (OIT), ocean island alkali basalts (OIA),
bon). AI = within‐plate alkali basalts; AII = within‐plate alkali basalts and
lcanic arc basalts; D = N‐type MORB and volcanic arc basalts. Symbols
rary.com]
738 SAKYI ET AL.
FIGURE 13 Plot of ɛNd (2.1 Ga) versus fractionation parameter (ƒSm/
Nd) for the metavolcanic and mafic intrusive rocks from the Kibi‐
Asamankese area. Fields of old (Archean) continental crust, MORB and
the arc rocks are from Roddaz, Debat, and Nikiéma, (2007) and the
references therein. Symbols are the same as in Figure 4a [Colour figure
can be viewed at wileyonlinelibrary.com]by arc rocks. This is consistent with the major and trace element data
and thus supports the arc affinity for the metavolcanic rocks and
mafic intrusions.
There have been disagreements regarding the tectonic setting of
the Paleoproterozoic rocks of the WAC. Previous studies have pro-
posed different models to support the tectonic setting of the rocks.
For example, Hirdes et al. (1996) have proposed more than one tec-
tonic setting for the evolution of the volcanic belts. The enrichment
in Ba, Th, and Sr, and negative Nb‐Ta, Zr‐Hf, Ce, and Ti anomalies
are typical characteristics of subduction‐related magmatism (Fitton,
James, Kempton, Ormerod, & Leeman, 1988; Saunders, Norry, &
Tarney, 1988), and the entire geochemical characteristics indicate an
island‐arc affinity for the metavolcanics and mafic rocks. Based on geo-
chemistry and field relations, our data is consistent with an island‐arc
setting (Attoh et al., 2006; Béziat et al., 2000; Dampare et al., 2008;
Sylvester & Attoh, 1992) than the proposed plume‐generated setting
(e.g., Abouchami et al., 1990) for the Paleoproterozoic rocks of the
WAC. We suggest that the subduction–accretion processes that
prevailed in the Paleoproterozoic terrane of the WAC during the
2.1–2.0 Ga Eburnean orogeny (Abouchami et al., 1990; Boher et al.,
1992; Liégois et al., 1991; Taylor et al., 1992) may have played a role
in the buildup of the Columbia supercontinent during Paleoproterozoic
(2.1–1.8 Ga) orogenic events.6 | CONCLUSIONS
Major and trace elements, and Sr‐Nd isotope compositions of
Paleoproterozoic metavolcanic rocks and mafic intrusions from Kibi‐
Winneba Greenstone Belt of the southeastern part of the WAC in
Ghana have been used to determine the petrogenesis and tectonicsetting of the rocks. The results show that fractional crystallization
may have played a vital role in the evolution of at least the
metavolcanics from the Birimian terrane of the WAC. The trace ele-
ments ratios, notably the Th/Yb and Nb/Yb suggest little or no crustal
contribution to the mantle source of the samples. Similarly, the εNd
values of −0.96 to +2.60 for the metavolcanic rocks and +0.46 to
+1.50 for the mafic intrusions suggest mantle‐derived magmas with
possible endogenic contamination. We, therefore, interpret the coex-
istence of the LREE‐enriched patterns of the studied tholeiites and
also εNd values (i.e., −0.96 to +2.60) as indicative of minor crustal
contamination in the petrogenesis of the tholeiites.
The Nd model ages of 2.24–2.51 Ga (TDM1) and 2.16–2.45 Ga
(TDM2), which are slightly higher than the formation age of the Birimian
(2.1 Ga), suggest possible contributions of a pre‐Birimian crustal mate-
rial (or Archean?) in the source material of the metavolcanics and mafic
intrusive rocks. Thus, the magma may have been isolated from its
source for a considerable period of time, thereby incorporating rela-
tively older materials. Also, the similar geochemical and isotopic char-
acteristics exhibited by the metavolcanics and mafic intrusive rocks
indicate that they were cogenetic. The isotopic results of this study
are consistent with an island arc tectonic setting arising from subduc-
tion–accretion processes, which are typical for the Paleoproterozoic
terranes of the WAC. The enrichment in Ba and Th and depletion in
Nb‐Ta, Zr‐Hf, and Ti of the rocks are typical characteristics of subduc-
tion‐related magmas. We speculate that the subduction–accretion
processes that prevailed in the Paleoproterozoic terrane of the WAC
during the 2.1–2.0 Ga Eburnean orogeny may have played a role in
the buildup of the Columbia supercontinent during the
Paleoproterozoic.
ACKNOWLEDGEMENTS
This research was financially supported by the National Natural
Science Foundation of China (Grant 41522203) and Youth Innovation
Promotion Association, Chinese Academy of Sciences (Grant
2016067) made available to BXS. Jin Xindi of theTrace Element Labo-
ratory, Institute of Geology and Geophysics, Chinese Academy of
Sciences is sincerely thanked for her assistance in the trace element
analysis. The authors express their sincere thanks to the entire staff
of the State Key Laboratory of Lithospheric Evolution, Institute of
Geology and Geophysics, Chinese Academy of Sciences for the various
roles they played in all the analyses. The authors would also like to
thank the editor and anonymous reviewers for their constructive com-
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How to cite this article: Sakyi PA, Anum S, Su B‐X, et al.
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