Provenance and Tectonic Setting of Late Proterozoic Buem Sandstones of Southeastern Ghana: Evidence from Geochemistry and Detrital Modes

Journal of African Earth Sciences 44 (2006) 85–96 www.elsevier.com/locate/jafrearsci

Provenance and tectonic setting of Late Proterozoic Buem sandstones of southeastern Ghana: Evidence from geochemistry and detrital modes

Shiloh Osae a, Daniel K. Asiedu b, Bruce Banoeng-Yakubo b, Christian Koeberl c,*, Samuel B. Dampare a

a National Nuclear Research Institute, Ghana Atomic Energy Commission, P.O. Box LG 80, Legon-Accra, Ghana b Department of Geology, University of Ghana, P.O. Box LG 58, Legon-Accra, Ghana c Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

Received 8 September 2004; received in revised form 3 October 2005; accepted 30 November 2005 Available online 9 January 2006

Abstract

The petrography, as well as major and trace element (including rare earth element) compositions of 10 sandstone samples from the Late Proterozoic Buem Structural Unit, southeast Ghana, have been investigated to determine their provenance and tectonic setting. The petrographic analysis has revealed that the sandstones are quartz-rich and were primarily derived from granitic and metamorphic basement rocks typical of a craton interior. The major and trace element compositions are comparable to average Proterozoic cratonic sandstones but with slight enrichment in high-ﬁeld strength elements (i.e., Zr, Hf, Ta, Nb) and slight depletion in ferromagnesian elements (e.g., Cr, Ni, V) with exception of Co which is unusually enriched in the sandstones. The geochemical data suggest that the Buem sandstones are dominated by mature, cratonic detritus deposited on a passive margin. Elemental ratios critical of provenance (La/Sc, Th/Sc, Cr/Th, Eu/Eu*, La/Lu) are similar to sediments derived from weathering of mostly felsic and not maﬁc rocks. The rather high Eu/Eu* ratios (0.69–1.09) suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. The granitoids of the Birimian Supergroup and/or the felsic gneisses of Birimian age exposed to the east and southeast of the Buem Formation appear the most likely source rocks. These results, therefore, support earlier studies that infer passive margin setting for the eastern margin of the West African Craton prior to the Pan-African orogeny. Ó 2005 Elsevier Ltd. All rights reserved.

Keywords: Ghana; Pan-African orogeny; Provenance; Sandstones; Petrography; Geochemistry

1. Introduction natures to sedimentary rocks in two distinct ways. Firstly, diﬀerent tectonic environments have distinctive The chemical composition of clastic sedimentary rocks provenance characteristics and, secondly, they are charac- is a function of a complex interplay of several variables, terized by distinctive sedimentary processes. Consequently including the nature of the source rocks, source area weath- sedimentary rocks have been used to constrain provenance ering and diagenesis (McLennan et al., 1993). However, the and to identify ancient tectonic settings (e.g., Dickinson tectonic setting of the sedimentary basins has been consid- et al., 1983; Bhatia, 1983; McLennan et al., 1993). ered as the overall primary control on the composition of The geology of Ghana (Fig. 1) can generally be divided sedimentary rocks (Dickinson, 1985). Plate tectonic pro- into four tectono-stratigraphic units: (1) an early Protero- cesses impart distinctive petrological and geochemical sig- zoic basement rocks (i.e., the Birimian and Tarkwaian); (2) late Proterozoic to early Paleozoic sedimentary cover (i.e., the Voltaian Group); the basement rocks and the sed- * Corresponding author. Tel.: +43 1 4277 53110; fax: +43 1 4277 9534. imentary cover form part of the West African craton; (3) E-mail address: [email protected] (C. Koeberl). mobile belt located in the eastern border of the craton

Fig. 1. Generalized tectono-stratigraphic map of Ghana.

which was developed during the Pan-African (around sandstones in order to infer their provenance and the tec- 600 Ma) orogeny (i.e., Dahomeyide Belt) and, (4) Late tonic setting of the BSU at the time of their deposition. Paleozoic to Mesozoic sedimentary basins. The Dahomey- ide belt consists, from west to east, three structural divi- 2. General geology of study area sions (Fig. 2; Affaton et al., 1980): the Buem Unit, the Togo Series (= Akwapimian or Atacora Unit) and, the Four major lithologic facies can be distinguished in the Dahomeyan Unit (or Benin Plain Unit). The Buem Struc- BSU in the study area (Fig. 3): (a) clastic sediments, (b) tural Unit (BSU) is composed predominantly of intercala- limestone and jasperoids, (c) volcanic rocks, and (d) serp- tions of volcanics and sediments, and has been dated entinites. The clastic rocks form the uppermost and lower- 624 Ma (Bozhko et al., 1971). A large part of the Dahome- most parts of the succession (Fig. 3). They comprise yan Unit, however, appears to comprise Birimian rocks sandstones, fine-grained quartzites, siltsones, and red remobilized during the Pan African orogeny (Grant, shales. The jasperoids are series of bedded, normally red 1969; Affaton et al., 1980; Agyei et al., 1987). cherts of massive appearance and sometimes brecciated. The geotectonic setting of the BSU is disputed and var- Some, however, may have formed by metasomatic alter- ious authors have given different interpretations: continen- ation of the clastic sediments, limestone and volcanics tal collision origin (e.g., Burke and Dewey, 1972, 1973), (Junner, 1940; Jones, 1990). The serpentinites are schistose intracratonic origin (e.g., Clifford, 1972), continental rift and massive in nature and rich in chromite. The volcanic origin (e.g., Attoh, 1990; Jones, 1990) and passive margin rocks consist predominantly of basalts, hawaiites, mugea- origin (e.g., Affaton et al., 1997). Most of these studies rite, and trachytes. on the original tectonic setting of the BSU have mainly The volcanic and the sedimentary rocks are interstrati- concentrated on the metavolcanic rocks (e.g., Affaton fied and, therefore, coeval. Jones (1990) suggested that et al., 1997; Attoh and Morgan, 2004) while the associated the two igneous suites (i.e., volcanics and serpentinites) sedimentary rocks which comprise the dominant unit have are unrelated; the volcanics were probably erupted during received less attention even though such rocks contain a a period of tension related to continental breakup at about wealth of information about provenance and tectonic set- 650 Ma, whereas the serpentinites mark a continental colli- ting (McLennan et al., 1990, 1993). As a contribution to sion at about 500 Ma. this debate on the tectonic setting of the BSU, we have The sandstones tend to crop out in lens shaped bodies a investigated the compositions of sandstones from the few hundred meters to a few kilometers long. The lenticular BSU exposed in the Anum, Kpando and surrounding areas shape of the sandstone bodies and paucity of sedimentary of southeast Ghana (Fig. 2). This contribution will, there- structures in the massive sandstones suggest their deposi- fore, examine the petrography and geochemistry of the tion as alluvial fan deposits (Jones, 1990). The associated S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 87

Fig. 2. Geological map of the study area. The location of this area is shown in Fig. 1.

shales are red and contain desiccation cracks and ripple marks indicating shallow water or subaerial deposition. The clastic sediments are, therefore, of continental origin. The BSU is considered as a tectonic and metamorphic lateral equivalents of the middle part of the Voltaian Group that has been dated 620–640 Ma (Grant, 1969; Affaton et al., 1980). However, K/Ar ages of three Buem volcanic specimens give a mean age of about 512 Ma (Jones, 1990), which is younger than the expected 650 Ma age for the deposition of the Buem Formation. Jones (1990) has suggested that this 500 Ma age coincide with metamorphic and metasomatic events that affected the Buem rocks after their deposition. Affaton et al. (1997), however, identified an earlier weak metamorphic imprint that is older than the Pan-African collision and may be coeval with the sedimentation age. This metamorphic imprint is marked by prehnite–pumpellyite facies metamorphism developed under temperatures of 200–300 °C. The alteration products of this metasomatic event include: (1) alteration of the volcanics to sericite, chlorite and carbonates; (2) formation of epidote/quartz veins in the volcanics; (3) intrusion of quartz veins into the sandstones, and (4) devel- Fig. 3. Lithologic column of the BSU in the study area. opment of jasper from a possible limestone precursor. 88 S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96

3. Sampling and methods sandstones can be classified by their matrix content and mineralogical content (Okada, 1971; Folk, 1974). On the Sandstone samples for this study were collected from basis of their mineralogical contents, the Buem sandstones outcrops in the Anum, Kpando, Nkonya and surrounding are classified as quartz arenite and feldspathic arenite areas (Figs. 2 and 3). Fresh rock exposures were scarce due (Fig. 4). The mean matrix content for the analyzed samples to intense tropical weathering. Ten of the least weathered is 3 vol%. The matrix of the feldspathic arenites is generally samples were selected for petrographic and geochemical composed of argillaceous materials (sericite and detrital study. They include four quartzite and six feldspathic sand- clay) that are squashed between framework grains. stone samples. The exact locations of the studied samples Pseudomatrix as defined by Dickinson (1970) and repre- are given in Table 1. senting altered malleable framework grains squashed Thin-sectioned point counting of the sandstones was between competent framework grains also occurs, but is used for quantitative compositional analysis. The modal generally rare. In contrast, the quartz arenites are typically analysis was performed by counting more than 300 points cemented with quartz, hematite, and sericite. per thin section, using the Gazzi–Dickinson point-counting Quartz is the most abundant framework grain in the method (Gazzi, 1966; Dickinson, 1970). This point- sandstones, constituting on average 87% of rock volume. counting method minimizes compositional dependence on The quartz grains are commonly sub-rounded to sub- grain size and, therefore, sandstones of different grain sizes angular in shape. Inclusions of chlorite and muscovite were can be compared (Ingersoll et al., 1984). observed in some thin-sections. Among quartz grains Qm Major and selected trace element (i.e., Rb, Sr, Y, Nb, (88 vol%) is dominant over Qp and most (ca. 60 vol%) Co, Ni, Cu, Zn, V, Cr, Ba) contents were determined on the 10 sandstone samples by X-ray fluorescence spectrom- etry (XRF) using standard techniques (see Reimold et al., 1994; for details on procedure and accuracy). All other trace and rare earth elements were determined using instrumental neutron activation analysis (INAA). Instrumenta- tion, sample preparation, data reduction techniques, and standards, precision and accuracy are described in Koeberl (1993).

4. Results

4.1. Petrography

The analyzed sandstone samples are moderately to well sorted, and the feldspathic sandstones are medium-grained, whereas the quartzites are generally fine-grained. The framework grains of the sandstones are composed of monocrystalline quartz (Qm), polycrystalline quartz (Qp), K-feldspar, plagioclase, and rock fragments. Quartz domi- Fig. 4. Mineralogical classification of the Buem sandstones (fields after nates over feldspar and rock fragments (Table 1). Detrital Okada, 1971). Q, Quartz; F, Feldspar; R, Rock fragments.

Table 1 Detrital modes from quartzites and feldspathic arenites of the Buem sandstones (in vol%) Sample Location coordinates Qm Qp K P Ls Lm M QFL (%) QmFLt (%) QFLQmFLt ANS16 0°09.090E6°29.700N 82.8 6.0 5.9 0.8 3.1 1.3 2.8 88.8 6.7 4.5 82.8 6.7 10.4 BLS10 0°16.650E6°52.560N 80.6 6.9 7.9 2.0 2.4 0.3 3.4 87.5 9.8 2.7 80.6 9.8 9.6 BLS01 0°20.260E7°11.880N 81.6 5.9 8.1 1.7 1.4 1.4 3.6 87.5 9.7 2.8 81.6 9.7 8.7 KPS02 0°16.770E6°59.820N 81.3 6.9 8.2 2.3 0.8 0.5 3.2 88.2 10.5 1.3 81.3 10.5 8.2 LMV10 0°08.400E6°25.920N 84.0 6.1 7.2 1.3 1.1 0.5 1.8 90.1 8.4 1.5 84.0 8.4 7.6 LJ11 0°08.330E6°26.220N 100.0 0.0 0.0 0.0 0.0 0.0 1.8 100.0 0.0 0.0 100.0 0.0 0.0 BMV15 0°08.540E6°25.920N 97.0 1.0 0.0 0.0 1.4 0.6 9.7 98.0 0.0 2.0 97.0 0.0 3.0 BMV08 0°20.260E7°11.880N 90.0 2.0 2.0 0.2 2.4 0.6 2.7 92.0 2.2 3.0 90.0 2.2 5.0 LT11 0°08.390E6°26.010N 100.0 0.0 0.0 0.0 0.0 0.0 3.2 100.0 0.0 0.0 100.0 0.0 0.0 AS06 0°19.930E7°04.300N 76.1 4.2 10.5 2.8 5.0 1.3 5.2 80.4 13.3 6.3 76.1 13.3 10.5 Qm = monocrystalline quartz; Qp = polycrystalline quartz; K = K-feldspar; P = plagioclase; Ls = sedimentary lithic fragments; Lm = metamorphic lithic fragments; M = matrix; F = K + P; L = Ls + Lm; Lt = Qp + Ls + Lm. S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 89

Qm has non-undulose extinction. Qp grains are composed 5. Provenance mainly of non-oriented crystallites, commonly two or three per grain, with straight to undulose extinction. 5.1. Source-area weathering All the analyzed feldspathic sandstone samples contain minor amounts of feldspar (F) grains (on average Alteration of igneous rocks during weathering results in 10 vol%). In contrast, the quartz arenites (i.e., quartzites) depletion of alkali and alkaline earth elements and prefer- lack feldspar. Feldspar grains are subangular and clear ential enrichment of Al2O3 in sediments. Therefore, the of inclusions. K-feldspar (K) dominates over plagioclase weathering history of ancient sedimentary rocks can be (K/F 0.88) and is mostly orthoclase and microperthite, evaluated in part by examining relationships among the and fewer microcline and sanidine grains. Plagioclase alkali and alkaline earth elements (Nesbitt and Young, grains show well-developed polysynthetic twinning. 1982). A good measure of the degree of chemical weather- The rock fragments are comparatively less abundant, ing can be obtained by calculation of the Chemical Index of and are dominantly of sedimentary and subordinately Alteration (CIA; Nesbitt and Young, 1982) and Plagioclase metamorphic origin. Compositionally, the most abundant Index of Alteration (PIA; Fedo et al., 1995). High CIA and types of lithic fragments are microcrystalline chert, quartz- PIA values (i.e., 75–100) indicate intensive weathering in ose sandstone, and argillites. the source area whereas low values (i.e., 60 or less) indicate A limited range of heavy minerals was observed in thin- low weathering in source area. section. The most common is zircon, which mostly occurs CIA and PIA values for the Buem sandstones are highly as silt-sized (<0.0625 mm) well-rounded grains. Other variable (i.e., 35–97), particularly for the quartz arenites heavy minerals species observed in thin-section include (Table 1). The high variations in CIA and PIA values tourmaline, garnet and rutile. may, however, be due to the low concentrations (sometimes below or ear detection limits) of the alkalis and alka- 4.2. Geochemistry line earth elements (Table 1) rather than variable degrees of source area weathering. Nevertheless, majority of the sam- The Buem sandstones (i.e., both feldspathic arenite and ples have CIA and PIA values greater than 60 indicating quartz arenite) have SiO2 contents between 89 and 96 wt% moderate to high weathering conditions in the source area. (i.e., quartz-rich following the criteria of Crook, 1974). The quartz arenites are depleted of K2O and TiO2 but 5.2. Tectonic setting enriched in Fe2O3 as compared to the feldspathic arenites (Table 2). Depletion of Na2O (<1 wt%) in both groups of The main assumption behind sandstone provenance sandstones can be attributed to the relatively small studies is that different tectonic settings contain character- amount of Na-rich plagioclase present, as shown by the istic rock types which, when eroded, produce sandstones petrographical data. K2O and Na2O contents and their with specific compositional ranges (Dickinson, 1985). The ratios (K2O/Na2O 1) are also consistent with the petro- analysis of sandstones with known provenance has been graphic observations, according to which K-feldspar dom- used to define these ranges from which the provenance of inates over plagioclase feldspar. Using the geochemical other samples can be deduced. classification diagram of Herron (1988), the Buem feld- Dickinson and co-workers have related detrital sand- spathic arenites are classified as subarkose and sublithare- stone compositions to major provenance types such as sta- nite (Fig. 5). ble cratons, basement uplifts, magmatic arcs and recycled In comparison with average upper continental crust orogens (Dickinson and Suczek, 1979; Dickinson et al., (UCC) the concentrations of most trace elements are gen- 1983). In the QFL and QmFLt ternary diagrams of erally low with exception of Co that is consistently enriched Dickinson et al. (1983) the analyzed samples plot exclu- relative to UCC for all the analyzed samples. The trace ele- sively in the craton interior field (Fig. 7). As pointed out ment abundances are, however, comparable to average by Dickinson et al. (1983), sandstones plotting in this field Proterozoic cratonic sandstones (PSS) but with slight are mature sandstones derived from relatively low-lying enrichment in high-field strength elements (Zr, Hf, Ta, granitoid and gneissic sources, supplemented by recycled Nb) and slight depletion in ferromagnesian elements with sands from associated platform or passive margin basins. exception of Co, which are of several orders enriched in Various workers (e.g., Bhatia, 1983; Roser and Korsch, the Buem sandstones (Fig. 6a). 1986; McLennan et al., 1990) have used the chemical com- All the analyzed samples display LREE enrichment rel- positions of sandstones to discriminate tectonic settings. ative to HREE with flat to slightly depleted HREE pat- Three tectonic settings—passive continental margin (PM), terns and variable but mostly negative Eu-anomalies active continental margin (ACM) and oceanic island-arc (Fig. 6b). In general, the Buem sandstones have similar (ARC)—are recognized on the K2O/Na2O–SiO2 discrimi- chondrite-normalized REE patterns similar to those of nation diagram of Roser and Korsch (1986). The fields PSS although most of the analyzed samples are depleted are based on ancient sandstone-mudstone pairs, verified in REE abundances relative to PSS due to quartz dilution against modern sediments from known tectonic settings. (Fig. 6b). On this diagram (Fig. 8), the Buem sandstones plot 90 S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96

Table 2 Chemical compositions of sandstones from the Buem Formation Feldspathic arenite Quartz arenite ANS16 BLS10 BLS01 KPS02 LMV10 ASO6 LJ11 BMV15 BMV08 LT11 wt%

SiO2 93.22 94.29 94.72 95.91 94.51 89.41 94.52 87.13 91.49 92.53 TiO2 0.11 0.12 0.11 0.12 0.07 0.21 0.01 0.09 0.05 0.01 Al2O3 2.99 2.78 2.81 2.10 1.85 4.82 0.20 3.65 1.57 0.17 Fe2O3 0.69 0.33 0.27 0.55 0.51 1.40 3.34 4.85 5.14 5.56 MnO 0.00 0.01 0.00 0.00 0.03 0.02 0.02 0.11 0.02 0.35 MgO 0.08 0.02 0.02 0.00 0.21 0.29 0.08 1.09 0.09 0.05 CaO 0.07 0.03 0.06 0.07 0.08 0.09 0.11 0.06 0.05 0.13

Na2O 0.05 0.08 0.09 0.36 0.31 0.19 0.05 0.01 0.24 0.05 K2O 0.31 1.07 1.01 0.86 0.54 1.13 0.00 0.00 0.00 0.00 P2O5 0.03 0.04 0.05 0.02 0.02 0.03 0.01 0.04 0.03 0.02 LOI 1.40 0.83 0.98 0.62 0.63 1.46 0.44 2.03 0.36 0.61 Total 98.96 99.60 100.13 100.61 98.77 99.05 98.79 99.06 99.04 99.48 CIA 84 67 67 56 60 74 97 76 35 60 PIA 92 90 87 62 66 88 97 76 35 64 ppm Sc 1.45 1.23 1.14 1.01 0.97 0.32 3.66 V 1913<12<12173926536342 Cr 30 25 19 32 24 47 19 21 28 55 Co 163 282 231 109 242 62 264 80 140 155 Ni <6 <6 <6 <6 <6 11 <6 47 <6 18 Cu 7 8 7 7 18 10 9 47 6 11 Zn 17 13 13 11 15.5 20 8.57 70.8 29 13 Rb 16 33 34 30 20 48 3 4 5 3 Sr 18 29 32 16 25 20 10 7 27 22 Ba 75 254 253 228 374 398 251 40 72 1298 Cs 0.38 0.5 0.46 0.6 0.54 0.38 0.22 Y678768496 4 Zr 109 165 165 166 79 112 17 36 17 17 Nb91011107966 4 5 Hf 2.32 4.25 4.25 4.92 1.79 0.22 1.00 Ta 0.76 1.47 0.91 0.62 1.26 1.22 0.50 Th 1.42 2.24 2.08 2.28 1.49 0.09 1.94 U < 0.5 0.38 < 0.1 < 0.6 0.54 0.2 1.02 La 9.90 13.8 13.8 7.26 5.11 1.15 6.21 Ce 14.3 21.7 22.40 13.40 9.50 2.05 10.8 Nd 5.12 10.1 10.3 5.91 3.64 0.80 4.22 Sm 0.80 1.87 1.89 1.03 0.73 0.18 1.15 Eu 0.29 0.42 0.44 0.20 0.20 0.04 0.31 Gd 0.82 1.36 1.48 1.01 0.70 0.10 1.00 Tb 0.13 0.21 0.23 0.18 0.12 0.02 0.17 Tm 0.07 0.13 0.13 0.10 0.06 0.01 0.10 Yb 0.46 0.68 0.71 0.73 0.35 0.06 0.61 Lu 0.07 0.10 0.11 0.11 0.05 0.01 0.09 Chemical Index of alteration (CIA, Nesbitt and Young, 1982) and plagioclase index of alteration (PIA, Fedo et al., 1995) calculated following the procedure given in Fedo et al. (1995).

exclusively in the PM ﬁeld. According to Roser and Korsch suggests that the Buem sandstones are derived from old (1986), PM sediments are largely quartz-rich sediments upper continental crust (McLennan et al., 1990). Accord- derived from plate interiors or stable continental areas ing to McLennan et al. (1990) this provenance component and deposited in intra-cratonic basins or on passive conti- constitutes old stable cratons and old continental founda- nental margins. tions of active tectonic settings. The Buem sandstones show the following chemical char- The volcanic rocks that are associated with the Buem acteristics: relatively uniform compositions, evolved major sandstones show strong alkali aﬃnities (Jones, 1990; Osae, element compositions (e.g., high SiO2/Al2O3,K2O/Na2O; unpublished data), and a continental rift setting is inferred Table 2), enrichments of normally incompatible over com- for their emplacement (Attoh, 1990; Jones, 1990). How- patible elements (e.g., LREE enrichment, high Th/Sc, La/ ever, a continental rift setting would produce immature Sc; Fig. 6a, Table 2), and high Rb/Sr ratios (>0.5). This clastic sediments resulting from rapid transportation and S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 91

burial, so as to preserve feldspar, particularly plagioclase. A continental rift setting should, therefore, have resulted in the associated sandstones plotting in the basement uplift field (Fig. 7), and within the ACM field in the K2O/Na2O– SiO2 diagram (Fig. 8). A continental rift setting, therefore, is not consistent with the petrographic and geochemical compositions of the sandstones. A passive margin or cratonic setting would be the most likely tectonic environment for the coeval volcanic and sedimentary activities. Alkaline igneous rocks, such as trachytes and phonolites, are not restricted to continental rifts but are also known to occur in cratons and oceanic islands (Condie, 1997). Also, a continental collision setting for the Buem Formation suggested as by Burke and Dewey (1972, 1973) is not compatible with Fig. 5. Chemical classification of the Buem feldspathic arenites (fields the compositions of the sandstones since such a setting after Herron, 1988). would have resulted in their plotting in the recycled orogen field (Fig. 7) and ACM field (Fig. 8). The compositions of the sandstones, however, are in agreement with other interpretations that suggest cratonic origin (Clifford, 1972) and passive margin origin (Affaton, 1990; Affaton et al., 1997) for the BSU.

5.3. Provenance

The qualitative petrography provides important information on the nature of the source area. The high propor- tion of quartz (and quartzose lithic fragments), as well as the dominance of K-feldspar over the more chemically unstable plagioclase in the Buem sandstones suggests that the source was exposed to prolonged weathering and that the sediment is at least partly multicyclic. This mineralogy is consistent with their derivation from granitic or acidic high-grade metamorphic rocks. However, the presence of rare rounded detrital quartz grains, sedimentary lithic frag- Fig. 6a. Distribution of high field strength elements, REE and ferromagnesian elements in sandstones from the Buem Formation. Data are ments, such as quartz arenite, and rounded grains of zircon normalized to average Proterozoic cratonic sandstone from Condie (1993). and tourmaline, suggest that a component of the prove- Data for Upper Continental Crust are from Condie (1993). nance is older (pre-existing) sedimentary rocks. All the studied samples contain both strained and unstrained quartz grains. Although the strained quartz could in part be due to the post-depositional effects of folding and metamorphism, the occurrence of both strain and unstrained quartz in suggest that some of the strain was inherited from the source area and, therefore, suggest a metamorphic and/ or plutonic source for the quartz grains (Young, 1976). This interpretation is compatible with granitic and/or metamorphic sources, adding weight to the interpretation that the Buem sandstones were derived from continental basement. Discriminant function analysis using major element compositions is another method for determining the provenance of sandstones (Roser and Korsch, 1988). The discriminant functions of Roser and Korsch (1988) were designed to discriminate between four sedimentary provenance fields. These are: mafic (P1); intermediate (P2); felsic Fig. 6b. Chondrite-normalized REE patterns in sandstones from the (P3); and recycled (P4). On this diagram, the Buem sand- Buem Formation. Normalizing values Taylor and McLennan (1985). Data stones plot in the P4 field (Fig. 9), supporting the interpre- for average Proterozoic sandstone after Condie (1993). tation that they were derived from granitic-gneissic or 92 S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96

Fig. 7. QFL and QmFLt plots. (a) and (c) Provenance ﬁelds of Dickinson et al. (1983). (b) and (d) sandstones from the Buem Formation. Deﬁnitions are given in Table 1.

Fig. 8. Provenance discrimination diagrams of Roser and Korsch (1986) Fig. 9. Provenance discrimination diagram of Roser and Korsch (1988) with sandstones from Buem Formation. Data for average Proterozoic with sandstones (ﬁlled circles) from the BSU. Also plotted for comparison sandstone are from Condie (1993). ARC, volcanic island arc; ACM, active are Upper Continental Crust (open circle) and average Proterozoic continental margin; PM, passive margin. sandstone (open square); Data from Condie (1993).

sedimentary source area, similar to PM-derived (Roser and The REE, Th and Sc are generally accepted as among the Korsch, 1988). The felsic and recycled source rocks for the most reliable indicators of sediment provenance because Buem sandstones is further supported by their high Th/Sc their distribution is less aﬀected by heavy-mineral fraction- and Zr/Sc ratios respectively (Fig. 10). ation than that of elements such as Zr, Hf, and Sn (Cullers S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 93

HREE patterns implies a granodiorite rather than a granite source (Condie, 1993; Cullers and Podkovyrov, 2000). However, the appreciably high Eu/Eu* values and the overall flat HREEs may suggest a component of mafic volcanic rocks (Fedo et al., 1996). The petrological and geochemical data indicate that the Buem sandstones were predominantly derived from a felsic igneous source with a component from pre-existing sedimentary source. To assess possible sources based on the above, we attempt to quantitatively model the provenance using selected three source end member components, i.e., Upper Proterozoic crust (UPC), felsic plutonic rocks (PG) and mafic-intermediate volcanic rocks. For the UPC end member we use the average of 24 Birimian meta- Fig. 10. Plot of Th/Sc versus Zr/Sc for sandstones from the BSU (after graywackes and phyllites from Asiedu et al. (2004) to rep- McLennan et al., 1993). The sandstones are enriched in zircon, due to sedimentary sorting and recycling. Average source rock compositions are resent the upper crustal composition at the time of of Proterozoic age (after Condie, 1993). BAS, basalt; AND, andesite; deposition of the Buem sandstones. Granitoids and gra- FVO, felsic volcanics; GRA, granite; TTG, tonalite–trondhjemite–grano- nitic gneisses of Eburnean age (Ho gneiss; Agyei et al., diorite; PSS, Proterozoic sandstone (Condie, 1993). 1987) are a potential source for the Buem Sandstones and, therefore, ideal for the PG end-member. However, et al., 1979; Taylor and McLennan, 1985). The REE and Th the chemical analyses for these felsic plutonic rocks are abundances are higher in felsic igneous rocks and in their not available so we use the average of five Birimian granit- weathering products, whereas Co, Sc, Ni, and Cr are more oid samples from the Ashanti greenstone belt (Kutu, concentrated in mafic than in felsic igneous rocks. The low unpublished data) to represent the PG end member. We concentrations of ferromagnesian trace elements such as Cr, use the average of four Buem volcanics samples from the Ni, Sc and V in the Buem sandstones (Table 2; Fig. 6a) indi- Kpando area (Osae, unpublished data) to represent the cate that very minimal mafic rocks were exposed in the mafic-intermediate end member. We apply the modeling source area. The unusual Co enrichment with respect to method of Fedo et al. (1996) that attempt to conserve mass average upper continental crust (Table 2) may suggest some balance amongst the relatively immobile REEs and in the input of mafic materials from the source terrane; however, Th/Sc ratio, which is a sensitive index of bulk composition the simultaneous depletion of Cr, Ni, and V suggests that (Taylor and McLennan, 1985). Parameters and results of other factors such as post-depositional alterations might the mixing calculations are shown in Table 4. have played a role in concentrating Ni in the sandstones. Using the source compositions listed in Table 4, average Furthermore, ratios such Eu/Eu*, (La/Lu)N, La/Sc, Buem feldsphatic arenite can be represented by a mixture Th/Sc, and Cr/Th are significantly different in mafic and fel- of 30% Birimian granitoids, 20% Birimian metasediments, sic source rocks and can, therefore, provide information and 50% Buem volcanics, whereas the average Buem about the provenance of sedimentary rocks (Amstrong- quartz arenite can be represented by 98% Birimian grani- Altrin et al., 2004). The Eu/Eu*, (La/Lu)N, La/Sc, Th/Sc, toids and 2% Buem volcanics. The modeled chondrite- and Cr/Th ratios of the Buem sandstones are similar to normalized REE patterns for both the feldspathic and those for sediments derived from felsic source rocks than quartz arenites are near identical to their respective average those for mafic source rocks (Table 3). The general lack of Buem sandstone (Fig. 11) but with higher REE abundances significant Eu-anomaly (average Eu/Eu* = 0.8) and flat obviously due to quartz dilution.

Table 3 Range of elemental ratios of Buem sandstones compared to the ratios in average Proterozoic sandstones, upper continental crust and sandstones derived from felsic rocks and maﬁc rocks Elemental ratio Range of Buem Range of sediments Range of sediments Average Proterozoic Upper Continental sandstonesa from felsic sourcesb from maﬁc sourcesb sandstonesc Crust (1.6–0.8 Ga)c La/Sc 1.70–12.1 2.50–16.3 0.43–0.86 4.21 1.91 Th/Sc 0.53–1.82 0.84–20.5 0.05–0.22 1.75 0.71 Cr/Th 5.74–21.1 4.00–15.0 25.0–500 5.71 4.46 Eu/Eu* 0.60–1.09 0.40–0.94 0.71–0.95 0.67 0.59

(La/Lu)N 6.85–14.7 3.00–27.0 1.10–7.00 8.07 7.21 a This study; Sample LJ11 is excluded because the concentrations of Sc and Th are near detection limits. b Amstrong-Altrin et al. (2004). c Condie (1993); Subscript N denotes chondrite-normalized value. 94

Table 4 Results from mixing calculations

Element Average Buem Mixing end members Mixing results 85–96 (2006) 44 Sciences Earth African of Journal / al. et Osae S. Feldspathic Quartz arenite Birimian Birimian Buem volcanicsc Feldspathic arenite Quartz arenite arenite granitoidsa metasedimentsb 30:20:50 98:0:2 ppm N ppm N ppm N ppm N ppm N ppm N ppm N La 9.97 27.18 3.68 10.03 13.46 36.67 23.69 65.95 147.2 401.0 82.35 224.6 16.13 43.95 Ce 16.26 16.99 6.43 6.71 31.78 33.20 49.11 50.51 287.1 300.0 162.9 170.0 36.88 38.54 Nd 7.01 9.86 2.51 3.53 13.29 18.69 19.01 26.74 111.0 156.1 63.29 89.01 15.24 21.44 Sm 1.26 5.47 0.67 2.88 2.51 10.87 4.46 18.65 20.65 89.39 11.97 51.69 2.87 12.44 Eu 0.31 3.56 0.18 2.01 0.67 7.73 1.31 15.10 4.50 51.67 2.71 31.17 0.75 8.61 Gd 1.07 3.51 0.55 1.80 2.15 7.03 4.26 17.25 18.35 59.97 10.67 35.54 2.47 8.08 Tb 0.17 3.00 0.10 1.64 0.34 5.82 0.69 13.15 2.77 47.76 1.62 28.26 0.39 6.66 Tm 0.10 2.75 0.06 1.54 0.17 4.71 1.86 52.11 0.20 5.65 Yb 0.59 2.36 0.34 1.35 1.12 4.51 1.36 5.42 10.75 43.35 5.98 24.11 1.31 5.28 Lu 0.09 2.32 0.05 1.31 0.19 4.86 0.20 5.18 1.52 39.76 0.85 22.37 0.21 5.55

(La/Sm)N 4.97 3.48 3.37 3.54 4.49 3.96 3.40 (Gd/Yb)N 1.49 1.33 1.56 3.18 1.38 1.80 1.56 (La/Yb)N 11.50 7.42 8.14 12.17 9.25 9.50 8.16 Eu/Eu* 0.81 0.88 0.88 0.84 0.71 0.79 0.88 Th/Sc 1.64 0.51 0.49 0.28 2.79 1.60 0.53 N denotes chondrite-normalized value. a Kutu (unpublished). b Asiedu et al. (2004). c Osae et al. (unpublished). S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 95

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