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 base- ment rocks typical of a craton interior. The major and trace element compositions are comparable to average Proterozoic cratonic sand- stones 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 sand- stones 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 The chemical composition of clastic sedimentary rocks is a function of a complex interplay of several variables, including the nature of the source rocks, source area weath- ering and diagenesis (McLennan et al., 1993). However, the tectonic setting of the sedimentary basins has been consid- ered as the overall primary control on the composition of sedimentary rocks (Dickinson, 1985). Plate tectonic pro- cesses impart distinctive petrological and geochemical sig- natures to sedimentary rocks in two distinct ways. Firstly, different tectonic environments have distinctive provenance characteristics and, secondly, they are charac- terized by distinctive sedimentary processes. Consequently sedimentary rocks have been used to constrain provenance and to identify ancient tectonic settings (e.g., Dickinson et al., 1983; Bhatia, 1983; McLennan et al., 1993). The geology of Ghana (Fig. 1) can generally be divided into four tectono-stratigraphic units: (1) an early Protero- 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- imentary cover form part of the West African craton; (3) mobile belt located in the eastern border of the craton 1464-343X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2005.11.009 * Corresponding author. Tel.: +43 1 4277 53110; fax: +43 1 4277 9534. E-mail address: [email protected](C. Koeberl). www.elsevier.com/locate/jafrearsci Journal of African Earth Sciences 44 (2006) 85–96
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www.elsevier.com/locate/jafrearsci
Journal of African Earth Sciences 44 (2006) 85–96
Provenance and tectonic setting of Late Proterozoic Buem sandstonesof 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, Ghanab 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 2005Available online 9 January 2006
Abstract
The petrography, as well as major and trace element (including rare earth element) compositions of 10 sandstone samples from theLate Proterozoic Buem Structural Unit, southeast Ghana, have been investigated to determine their provenance and tectonic setting. Thepetrographic analysis has revealed that the sandstones are quartz-rich and were primarily derived from granitic and metamorphic base-ment rocks typical of a craton interior. The major and trace element compositions are comparable to average Proterozoic cratonic sand-stones 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 sand-stones 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 oldupper continental crust. The granitoids of the Birimian Supergroup and/or the felsic gneisses of Birimian age exposed to the east andsoutheast of the Buem Formation appear the most likely source rocks. These results, therefore, support earlier studies that infer passivemargin setting for the eastern margin of the West African Craton prior to the Pan-African orogeny.� 2005 Elsevier Ltd. All rights reserved.
The chemical composition of clastic sedimentary rocksis a function of a complex interplay of several variables,including the nature of the source rocks, source area weath-ering and diagenesis (McLennan et al., 1993). However, thetectonic setting of the sedimentary basins has been consid-ered as the overall primary control on the composition ofsedimentary rocks (Dickinson, 1985). Plate tectonic pro-cesses impart distinctive petrological and geochemical sig-
1464-343X/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.
natures to sedimentary rocks in two distinct ways.Firstly, different tectonic environments have distinctiveprovenance characteristics and, secondly, they are charac-terized by distinctive sedimentary processes. Consequentlysedimentary rocks have been used to constrain provenanceand to identify ancient tectonic settings (e.g., Dickinsonet al., 1983; Bhatia, 1983; McLennan et al., 1993).
The geology of Ghana (Fig. 1) can generally be dividedinto four tectono-stratigraphic units: (1) an early Protero-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-imentary cover form part of the West African craton; (3)mobile belt located in the eastern border of the craton
Fig. 1. Generalized tectono-stratigraphic map of Ghana.
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which was developed during the Pan-African (around600 Ma) orogeny (i.e., Dahomeyide Belt) and, (4) LatePaleozoic to Mesozoic sedimentary basins. The Dahomey-ide belt consists, from west to east, three structural divi-sions (Fig. 2; Affaton et al., 1980): the Buem Unit, theTogo Series (= Akwapimian or Atacora Unit) and, theDahomeyan Unit (or Benin Plain Unit). The Buem Struc-tural Unit (BSU) is composed predominantly of intercala-tions of volcanics and sediments, and has been dated624 Ma (Bozhko et al., 1971). A large part of the Dahome-yan Unit, however, appears to comprise Birimian rocksremobilized during the Pan African orogeny (Grant,1969; Affaton et al., 1980; Agyei et al., 1987).
The geotectonic setting of the BSU is disputed and var-ious authors have given different interpretations: continen-tal collision origin (e.g., Burke and Dewey, 1972, 1973),intracratonic origin (e.g., Clifford, 1972), continental riftorigin (e.g., Attoh, 1990; Jones, 1990) and passive marginorigin (e.g., Affaton et al., 1997). Most of these studieson the original tectonic setting of the BSU have mainlyconcentrated on the metavolcanic rocks (e.g., Affatonet al., 1997; Attoh and Morgan, 2004) while the associatedsedimentary rocks which comprise the dominant unit havereceived less attention even though such rocks contain awealth of information about provenance and tectonic set-ting (McLennan et al., 1990, 1993). As a contribution tothis debate on the tectonic setting of the BSU, we haveinvestigated the compositions of sandstones from theBSU exposed in the Anum, Kpando and surrounding areasof southeast Ghana (Fig. 2). This contribution will, there-fore, examine the petrography and geochemistry of the
sandstones in order to infer their provenance and the tec-tonic setting of the BSU at the time of their deposition.
2. General geology of study area
Four major lithologic facies can be distinguished in theBSU in the study area (Fig. 3): (a) clastic sediments, (b)limestone and jasperoids, (c) volcanic rocks, and (d) serp-entinites. The clastic rocks form the uppermost and lower-most parts of the succession (Fig. 3). They comprisesandstones, fine-grained quartzites, siltsones, and redshales. The jasperoids are series of bedded, normally redcherts of massive appearance and sometimes brecciated.Some, however, may have formed by metasomatic alter-ation of the clastic sediments, limestone and volcanics(Junner, 1940; Jones, 1990). The serpentinites are schistoseand massive in nature and rich in chromite. The volcanicrocks consist predominantly of basalts, hawaiites, mugea-rite, and trachytes.
The volcanic and the sedimentary rocks are interstrati-fied and, therefore, coeval. Jones (1990) suggested thatthe two igneous suites (i.e., volcanics and serpentinites)are unrelated; the volcanics were probably erupted duringa period of tension related to continental breakup at about650 Ma, whereas the serpentinites mark a continental colli-sion at about 500 Ma.
The sandstones tend to crop out in lens shaped bodies afew hundred meters to a few kilometers long. The lenticularshape of the sandstone bodies and paucity of sedimentarystructures in the massive sandstones suggest their deposi-tion as alluvial fan deposits (Jones, 1990). The associated
Fig. 2. Geological map of the study area. The location of this area is shown in Fig. 1.
Fig. 3. Lithologic column of the BSU in the study area.
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shales are red and contain desiccation cracks and ripplemarks indicating shallow water or subaerial deposition.The clastic sediments are, therefore, of continental origin.
The BSU is considered as a tectonic and metamorphiclateral equivalents of the middle part of the VoltaianGroup that has been dated 620–640 Ma (Grant, 1969;Affaton et al., 1980). However, K/Ar ages of three Buemvolcanic specimens give a mean age of about 512 Ma(Jones, 1990), which is younger than the expected 650 Maage for the deposition of the Buem Formation. Jones(1990) has suggested that this 500 Ma age coincide withmetamorphic and metasomatic events that affected theBuem rocks after their deposition. Affaton et al. (1997),however, identified an earlier weak metamorphic imprintthat is older than the Pan-African collision and may be coe-val with the sedimentation age. This metamorphic imprintis marked by prehnite–pumpellyite facies metamorphismdeveloped under temperatures of 200–300 �C. The alter-ation products of this metasomatic event include: (1) alter-ation 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-opment of jasper from a possible limestone precursor.
Fig. 4. Mineralogical classification of the Buem sandstones (fields afterOkada, 1971). Q, Quartz; F, Feldspar; R, Rock fragments.
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3. Sampling and methods
Sandstone samples for this study were collected fromoutcrops in the Anum, Kpando, Nkonya and surroundingareas (Figs. 2 and 3). Fresh rock exposures were scarce dueto intense tropical weathering. Ten of the least weatheredsamples were selected for petrographic and geochemicalstudy. They include four quartzite and six feldspathic sand-stone samples. The exact locations of the studied samplesare given in Table 1.
Thin-sectioned point counting of the sandstones wasused for quantitative compositional analysis. The modalanalysis was performed by counting more than 300 pointsper thin section, using the Gazzi–Dickinson point-countingmethod (Gazzi, 1966; Dickinson, 1970). This point-counting method minimizes compositional dependence ongrain size and, therefore, sandstones of different grain sizescan be compared (Ingersoll et al., 1984).
Major and selected trace element (i.e., Rb, Sr, Y, Nb,Co, Ni, Cu, Zn, V, Cr, Ba) contents were determined onthe 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 othertrace and rare earth elements were determined using instru-mental neutron activation analysis (INAA). Instrumenta-tion, sample preparation, data reduction techniques, andstandards, precision and accuracy are described in Koeberl(1993).
4. Results
4.1. Petrography
The analyzed sandstone samples are moderately to wellsorted, and the feldspathic sandstones are medium-grained,whereas the quartzites are generally fine-grained. Theframework grains of the sandstones are composed ofmonocrystalline quartz (Qm), polycrystalline quartz (Qp),K-feldspar, plagioclase, and rock fragments. Quartz domi-nates over feldspar and rock fragments (Table 1). Detrital
Table 1Detrital modes from quartzites and feldspathic arenites of the Buem sandston
Qm = monocrystalline quartz; Qp = polycrystalline quartz; K = K-feldspar; Pfragments; M = matrix; F = K + P; L = Ls + Lm; Lt = Qp + Ls + Lm.
sandstones can be classified by their matrix content andmineralogical content (Okada, 1971; Folk, 1974). On thebasis of their mineralogical contents, the Buem sandstonesare classified as quartz arenite and feldspathic arenite(Fig. 4). The mean matrix content for the analyzed samplesis 3 vol%. The matrix of the feldspathic arenites is generallycomposed of argillaceous materials (sericite and detritalclay) that are squashed between framework grains.Pseudomatrix as defined by Dickinson (1970) and repre-senting altered malleable framework grains squashedbetween competent framework grains also occurs, but isgenerally rare. In contrast, the quartz arenites are typicallycemented with quartz, hematite, and sericite.
Quartz is the most abundant framework grain in thesandstones, constituting on average 87% of rock volume.The quartz grains are commonly sub-rounded to sub-angular in shape. Inclusions of chlorite and muscovite wereobserved in some thin-sections. Among quartz grains Qm(88 vol%) is dominant over Qp and most (ca. 60 vol%)
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Qm has non-undulose extinction. Qp grains are composedmainly of non-oriented crystallites, commonly two or threeper grain, with straight to undulose extinction.
All the analyzed feldspathic sandstone samples containminor amounts of feldspar (F) grains (on average10 vol%). In contrast, the quartz arenites (i.e., quartzites)lack feldspar. Feldspar grains are subangular and clearof inclusions. K-feldspar (K) dominates over plagioclase(K/F � 0.88) and is mostly orthoclase and microperthite,and fewer microcline and sanidine grains. Plagioclasegrains show well-developed polysynthetic twinning.
The rock fragments are comparatively less abundant,and are dominantly of sedimentary and subordinatelymetamorphic origin. Compositionally, the most abundanttypes of lithic fragments are microcrystalline chert, quartz-ose sandstone, and argillites.
A limited range of heavy minerals was observed in thin-section. The most common is zircon, which mostly occursas silt-sized (<0.0625 mm) well-rounded grains. Otherheavy minerals species observed in thin-section includetourmaline, garnet and rutile.
4.2. Geochemistry
The Buem sandstones (i.e., both feldspathic arenite andquartz arenite) have SiO2 contents between 89 and 96 wt%(i.e., quartz-rich following the criteria of Crook, 1974).The quartz arenites are depleted of K2O and TiO2 butenriched in Fe2O3 as compared to the feldspathic arenites(Table 2). Depletion of Na2O (<1 wt%) in both groups ofsandstones can be attributed to the relatively smallamount of Na-rich plagioclase present, as shown by thepetrographical data. K2O and Na2O contents and theirratios (K2O/Na2O � 1) are also consistent with the petro-graphic observations, according to which K-feldspar dom-inates over plagioclase feldspar. Using the geochemicalclassification diagram of Herron (1988), the Buem feld-spathic arenites are classified as subarkose and sublithare-nite (Fig. 5).
In comparison with average upper continental crust(UCC) the concentrations of most trace elements are gen-erally low with exception of Co that is consistently enrichedrelative to UCC for all the analyzed samples. The trace ele-ment abundances are, however, comparable to averageProterozoic cratonic sandstones (PSS) but with slightenrichment in high-field strength elements (Zr, Hf, Ta,Nb) and slight depletion in ferromagnesian elements withexception of Co, which are of several orders enriched inthe Buem sandstones (Fig. 6a).
All the analyzed samples display LREE enrichment rel-ative to HREE with flat to slightly depleted HREE pat-terns and variable but mostly negative Eu-anomalies(Fig. 6b). In general, the Buem sandstones have similarchondrite-normalized REE patterns similar to those ofPSS although most of the analyzed samples are depletedin REE abundances relative to PSS due to quartz dilution(Fig. 6b).
5. Provenance
5.1. Source-area weathering
Alteration of igneous rocks during weathering results indepletion of alkali and alkaline earth elements and prefer-ential enrichment of Al2O3 in sediments. Therefore, theweathering history of ancient sedimentary rocks can beevaluated in part by examining relationships among thealkali and alkaline earth elements (Nesbitt and Young,1982). A good measure of the degree of chemical weather-ing can be obtained by calculation of the Chemical Index ofAlteration (CIA; Nesbitt and Young, 1982) and PlagioclaseIndex of Alteration (PIA; Fedo et al., 1995). High CIA andPIA values (i.e., 75–100) indicate intensive weathering inthe source area whereas low values (i.e., 60 or less) indicatelow weathering in source area.
CIA and PIA values for the Buem sandstones are highlyvariable (i.e., 35–97), particularly for the quartz arenites(Table 1). The high variations in CIA and PIA valuesmay, however, be due to the low concentrations (some-times below or ear detection limits) of the alkalis and alka-line earth elements (Table 1) rather than variable degrees ofsource area weathering. Nevertheless, majority of the sam-ples have CIA and PIA values greater than 60 indicatingmoderate to high weathering conditions in the source area.
5.2. Tectonic setting
The main assumption behind sandstone provenancestudies is that different tectonic settings contain character-istic rock types which, when eroded, produce sandstoneswith specific compositional ranges (Dickinson, 1985). Theanalysis of sandstones with known provenance has beenused to define these ranges from which the provenance ofother samples can be deduced.
Dickinson and co-workers have related detrital sand-stone compositions to major provenance types such as sta-ble cratons, basement uplifts, magmatic arcs and recycledorogens (Dickinson and Suczek, 1979; Dickinson et al.,1983). In the QFL and QmFLt ternary diagrams ofDickinson et al. (1983) the analyzed samples plot exclu-sively in the craton interior field (Fig. 7). As pointed outby Dickinson et al. (1983), sandstones plotting in this fieldare mature sandstones derived from relatively low-lyinggranitoid and gneissic sources, supplemented by recycledsands from associated platform or passive margin basins.
Various workers (e.g., Bhatia, 1983; Roser and Korsch,1986; McLennan et al., 1990) have used the chemical com-positions of sandstones to discriminate tectonic settings.Three tectonic settings—passive continental margin (PM),active continental margin (ACM) and oceanic island-arc(ARC)—are recognized on the K2O/Na2O–SiO2 discrimi-nation diagram of Roser and Korsch (1986). The fieldsare based on ancient sandstone-mudstone pairs, verifiedagainst modern sediments from known tectonic settings.On this diagram (Fig. 8), the Buem sandstones plot
Table 2Chemical compositions of sandstones from the Buem Formation
Chemical Index of alteration (CIA, Nesbitt and Young, 1982) and plagioclase index of alteration (PIA, Fedo et al., 1995) calculated following theprocedure given in Fedo et al. (1995).
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exclusively in the PM field. According to Roser and Korsch(1986), PM sediments are largely quartz-rich sedimentsderived from plate interiors or stable continental areasand deposited in intra-cratonic basins or on passive conti-nental margins.
The Buem sandstones show the following chemical char-acteristics: relatively uniform compositions, evolved majorelement compositions (e.g., high SiO2/Al2O3, K2O/Na2O;Table 2), enrichments of normally incompatible over com-patible elements (e.g., LREE enrichment, high Th/Sc, La/Sc; Fig. 6a, Table 2), and high Rb/Sr ratios (>0.5). This
suggests that the Buem sandstones are derived from oldupper continental crust (McLennan et al., 1990). Accord-ing to McLennan et al. (1990) this provenance componentconstitutes old stable cratons and old continental founda-tions of active tectonic settings.
The volcanic rocks that are associated with the Buemsandstones show strong alkali affinities (Jones, 1990; Osae,unpublished data), and a continental rift setting is inferredfor their emplacement (Attoh, 1990; Jones, 1990). How-ever, a continental rift setting would produce immatureclastic sediments resulting from rapid transportation and
Fig. 5. Chemical classification of the Buem feldspathic arenites (fieldsafter Herron, 1988).
Fig. 6a. Distribution of high field strength elements, REE and ferromag-nesian elements in sandstones from the Buem Formation. Data arenormalized to average Proterozoic cratonic sandstone from Condie (1993).Data for Upper Continental Crust are from Condie (1993).
Fig. 6b. Chondrite-normalized REE patterns in sandstones from theBuem Formation. Normalizing values Taylor and McLennan (1985). Datafor average Proterozoic sandstone after Condie (1993).
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burial, so as to preserve feldspar, particularly plagioclase.A continental rift setting should, therefore, have resultedin the associated sandstones plotting in the basement upliftfield (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 geochemicalcompositions of the sandstones. A passive margin or cra-tonic setting would be the most likely tectonic environmentfor the coeval volcanic and sedimentary activities. Alkalineigneous rocks, such as trachytes and phonolites, are notrestricted to continental rifts but are also known to occurin cratons and oceanic islands (Condie, 1997). Also, a con-tinental collision setting for the Buem Formation suggestedas by Burke and Dewey (1972, 1973) is not compatible withthe compositions of the sandstones since such a settingwould have resulted in their plotting in the recycled orogenfield (Fig. 7) and ACM field (Fig. 8). The compositions ofthe sandstones, however, are in agreement with other inter-pretations that suggest cratonic origin (Clifford, 1972) andpassive margin origin (Affaton, 1990; Affaton et al., 1997)for the BSU.
5.3. Provenance
The qualitative petrography provides important infor-mation on the nature of the source area. The high propor-tion of quartz (and quartzose lithic fragments), as well asthe dominance of K-feldspar over the more chemicallyunstable plagioclase in the Buem sandstones suggests thatthe source was exposed to prolonged weathering and thatthe sediment is at least partly multicyclic. This mineralogyis consistent with their derivation from granitic or acidichigh-grade metamorphic rocks. However, the presence ofrare rounded detrital quartz grains, sedimentary lithic frag-ments, such as quartz arenite, and rounded grains of zirconand tourmaline, suggest that a component of the prove-nance is older (pre-existing) sedimentary rocks. All thestudied samples contain both strained and unstrainedquartz grains. Although the strained quartz could in partbe due to the post-depositional effects of folding and meta-morphism, the occurrence of both strain and unstrainedquartz in suggest that some of the strain was inherited fromthe source area and, therefore, suggest a metamorphic and/or plutonic source for the quartz grains (Young, 1976).This interpretation is compatible with granitic and/ormetamorphic sources, adding weight to the interpretationthat the Buem sandstones were derived from continentalbasement.
Discriminant function analysis using major elementcompositions is another method for determining the prov-enance of sandstones (Roser and Korsch, 1988). The dis-criminant functions of Roser and Korsch (1988) weredesigned to discriminate between four sedimentary prove-nance fields. These are: mafic (P1); intermediate (P2); felsic(P3); and recycled (P4). On this diagram, the Buem sand-stones plot in the P4 field (Fig. 9), supporting the interpre-tation that they were derived from granitic-gneissic or
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 aregiven in Table 1.
Fig. 8. Provenance discrimination diagrams of Roser and Korsch (1986)with sandstones from Buem Formation. Data for average Proterozoicsandstone are from Condie (1993). ARC, volcanic island arc; ACM, activecontinental margin; PM, passive margin.
Fig. 9. Provenance discrimination diagram of Roser and Korsch (1988)with sandstones (filled circles) from the BSU. Also plotted for comparisonare Upper Continental Crust (open circle) and average Proterozoicsandstone (open square); Data from Condie (1993).
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sedimentary source area, similar to PM-derived (Roser andKorsch, 1988). The felsic and recycled source rocks for theBuem sandstones is further supported by their high Th/Scand Zr/Sc ratios respectively (Fig. 10).
The REE, Th and Sc are generally accepted as among themost reliable indicators of sediment provenance becausetheir distribution is less affected by heavy-mineral fraction-ation than that of elements such as Zr, Hf, and Sn (Cullers
Fig. 10. Plot of Th/Sc versus Zr/Sc for sandstones from the BSU (afterMcLennan et al., 1993). The sandstones are enriched in zircon, due tosedimentary sorting and recycling. Average source rock compositions areof Proterozoic age (after Condie, 1993). BAS, basalt; AND, andesite;FVO, felsic volcanics; GRA, granite; TTG, tonalite–trondhjemite–grano-diorite; PSS, Proterozoic sandstone (Condie, 1993).
S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 93
et al., 1979; Taylor andMcLennan, 1985). The REE and Thabundances are higher in felsic igneous rocks and in theirweathering products, whereas Co, Sc, Ni, and Cr are moreconcentrated in mafic than in felsic igneous rocks. The lowconcentrations of ferromagnesian trace elements such as Cr,Ni, Sc and V in the Buem sandstones (Table 2; Fig. 6a) indi-cate that very minimal mafic rocks were exposed in thesource area. The unusual Co enrichment with respect toaverage upper continental crust (Table 2) may suggest someinput of mafic materials from the source terrane; however,the simultaneous depletion of Cr, Ni, and V suggests thatother factors such as post-depositional alterations mighthave played a role in concentrating Ni in the sandstones.Furthermore, ratios such Eu/Eu*, (La/Lu)N, La/Sc,Th/Sc, and Cr/Th are significantly different in mafic and fel-sic source rocks and can, therefore, provide informationabout the provenance of sedimentary rocks (Amstrong-Altrin et al., 2004). The Eu/Eu*, (La/Lu)N, La/Sc, Th/Sc,and Cr/Th ratios of the Buem sandstones are similar tothose for sediments derived from felsic source rocks thanthose for mafic source rocks (Table 3). The general lack ofsignificant Eu-anomaly (average Eu/Eu* = 0.8) and flat
Table 3Range of elemental ratios of Buem sandstones compared to the ratios in averafrom felsic rocks and mafic rocks
a This study; Sample LJ11 is excluded because the concentrations of Sc andb Amstrong-Altrin et al. (2004).c Condie (1993); Subscript N denotes chondrite-normalized value.
HREE patterns implies a granodiorite rather than a granitesource (Condie, 1993; Cullers and Podkovyrov, 2000).However, the appreciably high Eu/Eu* values and the over-all flat HREEs may suggest a component of mafic volcanicrocks (Fedo et al., 1996).
The petrological and geochemical data indicate that theBuem sandstones were predominantly derived from a felsicigneous source with a component from pre-existing sedi-mentary source. To assess possible sources based on theabove, we attempt to quantitatively model the provenanceusing selected three source end member components, i.e.,Upper Proterozoic crust (UPC), felsic plutonic rocks(PG) and mafic-intermediate volcanic rocks. For theUPC end member we use the average of 24 Birimian meta-graywackes and phyllites from Asiedu et al. (2004) to rep-resent the upper crustal composition at the time ofdeposition of the Buem sandstones. Granitoids and gra-nitic gneisses of Eburnean age (Ho gneiss; Agyei et al.,1987) are a potential source for the Buem Sandstonesand, therefore, ideal for the PG end-member. However,the chemical analyses for these felsic plutonic rocks arenot available so we use the average of five Birimian granit-oid samples from the Ashanti greenstone belt (Kutu,unpublished data) to represent the PG end member. Weuse the average of four Buem volcanics samples from theKpando area (Osae, unpublished data) to represent themafic-intermediate end member. We apply the modelingmethod of Fedo et al. (1996) that attempt to conserve massbalance amongst the relatively immobile REEs and in theTh/Sc ratio, which is a sensitive index of bulk composition(Taylor and McLennan, 1985). Parameters and results ofthe mixing calculations are shown in Table 4.
Using the source compositions listed in Table 4, averageBuem feldsphatic arenite can be represented by a mixtureof 30% Birimian granitoids, 20% Birimian metasediments,and 50% Buem volcanics, whereas the average Buemquartz arenite can be represented by 98% Birimian grani-toids and 2% Buem volcanics. The modeled chondrite-normalized REE patterns for both the feldspathic andquartz arenites are near identical to their respective averageBuem sandstone (Fig. 11) but with higher REE abundancesobviously due to quartz dilution.
ge Proterozoic sandstones, upper continental crust and sandstones derived
N denotes chondrite-normalized value.a Kutu (unpublished).b Asiedu et al. (2004).c Osae et al. (unpublished).
94S.Osaeet
al./JournalofAfrica
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ces44(2006)85–96
Fig. 11. Results from mixing calculations for the REEs of the Buemfeldspathic arenite (FAx:y:z) and quartz arenite (QAx:y:z) plotted withaverage Buem feldspathic arenite (FA) and quartz arenite (QA). See Tablefor mixing parameters.
S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 95
6. Conclusions
The provenance of the Buem sandstones of southeasternGhana has been assessed using integrated petrographicaland geochemical studies, the results which are broadly inagreement. This approach has revealed that the Buemsandstones were primarily derived from felsic continentalsources typical of a craton interior. The provenance char-acteristics suggest that the Buem sandstones were depositedon a passive margin that received large amounts of maturedetritus from the hinterland areas. This interpretation is inagreement with paleomagnetic data that do not supportlarge-scale relative plate motions within Africa in Protero-zoic–Paleozoic time (Piper et al., 1973). Based on Th/Scratios and REE patterns, the feldsphatic arenites of Buemsandstones can be modeled by a mixture of 30% Birimiangranitoids, 20% Birimian metasediments, and 50% Buemvolcanics, and the quartz arenite by 98% Birimian grani-toids and 2% Buem volcanics.
Acknowledgements
The NAA analyses were carried out by the first author(S. Osae) at the University of Vienna, during a fellowshipprogram that was sponsored by the International AtomicEnergy Agency. Mr. B. Bodiselitsch (Univ. Vienna) helpedwith the analyses. Laboratory work in Vienna was alsosupported by the Austrian Science Foundation FWF (pro-ject P17194-N10). We are grateful for the constructive re-views by K. Attoh, C.I. Chalokwu, and B.P. Roser, aswell as for editorial comments by P. Eriksson, which helpedto improve the manuscript.
References
Affaton, P., 1990. Le Bassin des Volta (Afrique de I’Ouest): une margepassive, d’age proterozoıque superieur, tectonisee au Panafrican
(600 + 50 Ma). Editions de I’ORSTOM, Collection Etudes et Theses,Paris.
Affaton, P., Aguirre, L., Menot, R.-P., 1997. Thermal and geodynamicsetting of the Buem volcanic rocks near Tiele, Northwest Benin, WestAfrica. Precambrian Res. 82, 191–209.
Affaton, P., Sougy, J., Trompette, R., 1980. The tectono-stratigraphicrelationships between the Upper Precambrian and Lower PaleozoicVolta Basin and the Pan-African Dohomeyide orogenic belt (WestAfrica). Am. J. Sci. 280, 224–248.
Agyei, E.K., van Landewijk, J.E.J.M., Armstrong, R.L., Harakal, J.E.,Scott, K.L., 1987. Rb–Sr and K–Ar geochronology of southeasternGhana. J. Afr. Earth Sci. 6, 153–161.
Amstrong-Altrin, J.S., Lee, Y.I., Verma, S.P., Ramasamy, S., 2004.Geochemistry of sandstones from the Upper Miocene KudankulamFormation, southern India: implication for provenance, weatheringand tectonic setting. J. Sediment. Res. 74, 285–297.
Asiedu, D.K., Dampare, S.B., Asamoah Sakyi, P., Banoeng-Yakubo, B.,Osae, S., Nyarko, B.J.B., Manu, J., 2004. Geochemistry of Palepro-terozoic metasedimentary rocks from the Birim diamondiferous field,southern Ghana: implications for provenance and crustal evolution atthe Archean–Proterozoic boundary. Geochem. J. 38, 215–228.
Attoh, K., 1990. Dahomeyides of Southeastern Ghana: Evidence foroceanic closure and crustal imbrication in a Pan-African orogen.Publication occasionnelle du C.I.F.E.G., BRGM, Orleans Tr. No. 21.
Attoh, K., Morgan, M., 2004. Geochemistry of high-pressure granulitesfrom the Pan-African Dahomeyide orogen, West Africa: constraintson the origin and composition of the lower crust. J. Afr. Earth Sci. 39,201–208.
Bhatia, M.R., 1983. Plate tectonics and geochemical composition ofsandstones. J. Geol. 91, 611–627.
Bozhko, N.A., Kazakov, G.A., Trofimov, D.M., Knorre, K.G., Gatinsky,Y.U.A., 1971. New absolute dating of West Africa glauconites. Acad.Nauk. SSSR, Doklady, Earth Science Section 198, 138–139 (AGItranslations).
Burke, K.C., Dewey, J.F., 1973. An outline of Precambrian platedevelopment. In: Tarling, D.M., Runcorm, S.K. (Eds.), Implicationof Continental Drift to the Earth Sciences, vol. 2. Academic Press,London, pp. 1035–1045.
Clifford, T.N., 1972. The evolution of the crust of Africa. Service Geol.Maroc., Notes Mem. No. 236, pp. 29–39.
Condie, K.C., 1993. Chemical composition and evolution of the uppercontinental crust: contrasting results from surface samples and shales.Chem. Geol. 104, 1–37.
Condie, K.C., 1997. Plate Tectonics and Crustal Evolution. Butterworth-Heinemann, Oxford.
Crook, K.A.W., 1974. Lithogenesis and geotectonics: the significance ofcompositional variation in flysch arenites (greywackes). Soc. Econ.Paleont. Mineral. Spec. Publ. No. 19, pp. 304–310.
Cullers, R.L., Chaudhuri, S., Kilbane, N., Koch, R., 1979. Rare-earths insize fractions and sedimentary rocks of Pennsylvanian–Permian agefrom the mid-continent of the USA. Geochim. Cosmochim. Acta 43,1285–1301.
Cullers, R.L., Podkovyrov, V.N., 2000. Geochemistry of the Mesoprote-rosoic Lakhanda shales in southern Yakutia, Russia: implications formineralogical and provenance control, and recycling. PrecambrianRes. 104, 77–93.
Dickinson, W.R., 1970. Interpreting detrital modes of greywacke andarkose. J. Sediment. Petrol. 40, 695–707.
Dickinson, W.R., 1985. Interpreting provenance relations from detritalmodes of sandstones. In: Zuffa, G. (Ed.), Provenance of Arenites.Reidel, Dordrecht, pp. 333–361.
Dickinson, W.R., Beard, L.S., Brakenridge, G.R., Erjavec, J.L., Ferguson,R.C., Inman, K.F., Knepp, R.A., Lindberg, F.A., Ryberg, P.T., 1983.Provenance of North American Phanerozoic sandstone in relation totectonic setting. Geol. Soc. Am. Bull. 94, 222–235.
96 S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96
Fedo, C.M., Eriksson, K.A., Krogstad, E.J., 1996. Geochemistry of shalesfrom the Archean (�3.0 Ga) Buhwa Greenstone Belt, Zimbabwe:implications for provenance and source-area weathering. Geochim.Cosmochim. Acta 60, 1751–1763.
Fedo, C.M., Nesbitt, H.W., Young, G.M., 1995. Unraveling the effects ofpotassium metasomatism in sedimentary rocks and paleosols, withimplications for paleoweathering conditions and provenance. Geology23, 921–924.
Gazzi, P., 1966. Le arenarie del flysch sopracretaceo dell’Appenninomodenese; correlazioni con il flysch di Monghidoro. Mineralog.Petrograph. Acta 12, 69–97.
Grant, N.K., 1969. The late Precambrian to early Paleozoic Pan-Africanorogeny in Ghana, Togo, Dahomey and Nigeria. Geol. Soc. Am. Bull.80, 45–55.
Herron, M.M., 1988. Geochemical classification of terrigenous sands andshales from core or log data. J. Sediment. Petrol. 58, 820–829.
Ingersoll, R.V., Bullard, T.F., Ford, R.L., Grimm, J.P., Pickle, J.D.,Sares, S.W., 1984. The effect of grain size on detrital modes: a test ofthe Gazzi-Dickinson point-counting method. J. Sediment. Petrol. 46,620–632.
Jones, W.B., 1990. The Buem volcanic and associated sedimentary rocks,Ghana: a field and geochemical investigation. J. Afr. Earth Sci. 11,373–383.
Junner, N.R., 1940. Geology of the Gold Coast and Western Togoland.Gold Coast Geol. Surv. Bull. 11.
Koeberl, C., 1993. Instrumental neutron activation analysis of geochem-ical and cosmochemical samples: a fast and proven method for smallsample analysis. J. Radioanal. Nucl. Chem. 168, 47–60.
McLennan, S.M., Hemming, S., McDaniel, D.K., Hanson, G.N., 1993.Geochemical approaches to sedimentation, provenance, and tectonics.In: Johnson, M.J., Basu, A., (Eds.), Processes Controlling theComposition of Clastic Sediments. Geol. Soc. Am. Spec. Paper 284,pp. 21–40.
McLennan, S.M., Taylor, S.R., McCulloch, M.T., Maynard, J.B., 1990.Geochemical and Nd–Sr isotopic composition of deep-sea turbidites:crustal evolution and plate tectonic associations. Geochim. Cosmo-chim. Acta. 54, 2015–2050.
Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and platemotions inferred from major element chemistry of lutites. Nature 299,715–717.
Okada, H., 1971. Classification of sandstones: analysis and proposals. J.Geol. 79, 509–525.
Piper, J.D.A., Briden, J.C., Lomax, K., 1973. Precambrian Africanand South American as a single continent. Nature 245, 244–248.
Roser, B.P., Korsch, R.J., 1986. Determination of tectonic setting ofsandstone-mudstone suites using SiO2 content and K2O/Na2O ratio. J.Geol. 94, 635–650.
Roser, B.P., Korsch, R.J., 1988. Provenance signatures of sandstone-mudstone suites determined using discriminant function analysis ofmajor-element data. Chem. Geol. 67, 119–139.
Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Com-position and Evolution. Blackwell Scientific, Oxford, p. 312.
Young, S.W., 1976. Petrographic textures of detrital polycrystalline quartzas an aid to interpreting crystalline source rocks. J. Sediment. Petrol.46, 595–603.
