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

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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 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 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 1464-343X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2005.11.009 86 S. Osae et al. / Journal of African Earth Sciences 44 (2006) 85–96 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 coe- val with the sedimentation age. This metamorphic imprint is marked by prehnite–pumpellyite facies metamorphism developed 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- 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.
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