Browsing by Author "Agra, N.A."
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Item Petrogenetic Evolution of the Eastern Buem Volcanic Rocks, South-Eastern Ghana(Science and Development (A Journal of the College of Basic and Applied Sciences (CBAS), University of Ghana), 2017-12) Agra, N.A.; Kwayisi, D.; Amponsah, P.O.; Dampare, S.B.; Asiedu, D.; Nude, P.M.Petrography, major, trace element and Rare Earth Element (REE) data are presented for volcanic rocks from the Eastern Buem Structural Unit (BSU) in south-eastern Ghana to constrain their petrogenesis and tectonic setting. The volcanic rocks are generally aphanitic, ophitic and massive, although some varieties exhibit weak foliation. They are primarily composed of plagioclase and pyroxene. These primary minerals have either partially or wholly altered to chlorite, epidote, sericite and opaque minerals. The basalts have low TiO2 (0.56– 1.19wt. %) content with fairly low magnesium numbers ranging from 38 to 55. They display flat to slightly depleted REE patterns and are mostly more enriched than chondrite. On the primitive mantle (PM) normalised spider diagram, they are variably enriched in Light Rare Earth Elements (LREE) and incompatible elements relative to normal mid-ocean ridge basalt (N-MORB), with similar Large-ion Lithophile Element (LILE) and High Field Strength Element (HFSE) patterns as enriched mid-ocean ridge basalt (E-MORB) but 10 fold more enriched than primitive mantle (PM). They show minor negative and positive Sr, positive Cs, Ba, Ta, La and Ce anomalies and minor negative Rb, Th, Zr and Ti anomalies. The basalts plot within the MORB mantle array on the Th/Yb versus the Nb/Yb diagram and have low Th/Nb ratios (0.07– 0.09) which indicate their derivation from asthenospheric sources with minimal or no contamination from crustal or subducted components. They show affinity to Enriched MORB on multi-trace element normalised diagrams. The E-MORB affinity of the basalts is also confirmed on Th–Hf–Nb, Y–La–Nb and V–Ti tectonic discrimination diagrams. These volcanic rocks may have erupted during rifting along the marginal basin of the WAC prior to peak collision during the Pan-African Orogeny.Item Provenance of shales and sandstones from the Devonian Accraian Group, southern Ghana(Korean Association of Geoscience Societies, 2018-06) Anani, C.Y.; Kwayisi, D.; Agra, N.A.; Asiedu, D.K.Petrographic and whole rock geochemical studies were carried out on sandstones and shales of the Devonian Accraian Group of southern Ghana to constrain their provenance and tectonic setting. Modal petrographic analysis on the sandstones show that the framework grains are sub-angular to rounded in shape. Upper continental crust normalized multi-element plots, as well as trace element ratios, suggests that the sandstones and shales were mainly derived from felsic and recycled sources. Modal mineralogy of the sandstones, and major and trace element geochemistry of sandstones and shales further suggest that they are of recycled orogenic provenance. Discriminant function based major-element diagram suggests that the Accraian sandstones and shales were deposited in a Passive margin setting. Weathering indices suggest high degree of source area weathering. It is of the view that the Accraian sandstones and shales, most probably, received sediment input mostly from the Paleoproterozoic Birimian metasedimentary rocks and their associated granitoids and/or the Neoproterozoic sedimentary rocks from the Volta Basin which are both positioned northwest and north, respectively, of the Accraian sedimentary rocks. © 2018, The Association of Korean Geoscience Societies and Springer-Verlag GmbH Germany, part of Springer Nature.Item Two suites of gabbros in the Buem Structural Unit, of the Pan-African Dahomeyide orogen, southeastern Ghana: Constraints from new field and geochemical data(Journal of African Earth Sciences, 2017) Kwayisi, D.; Agra, N.A.; Dampare, S.B.; Asiedu, D.K.; Amponsah, P.O.; Nude, P.M.The Buem Structural Unit (BSU) of the Pan-African Dahomeyide orogenic belt, southeastern Ghana, is characterized by the occurrence of clastic and chemical sedimentary rocks, volcanic rocks and mafic-ultramafic rocks. The mafic rocks, comprising mainly gabbros occur in smaller proportion within the BSU. Representative samples of the gabbros have been analyzed petrographically and geochemically to infer their mode of emplacement, petrogenesis and tectonic setting. Two distinct gabbro types have been identified from petrography and whole-rock major and trace elements geochemistry, namely the B1 and B2 gabbros. The gabbros are generally phaneritic, holocrystalline and composed of plagioclase and clinopyronexe which have been replaced either completely or partially by sericite, chlorite and epidote. On the whole, B1 gabbros are deformed, altered, alkaline in nature, show no significant effect of crustal contamination and has affinity to OIB/E-MORB. The B2 gabbros, are relatively less deformed, subalkaline, akin to N-MORB and show arc signatures with minimal crustal contamination. The geochemical characteristics of the B1 gabbros are similar to the Buem volcanic rocks. The similarity of the B1 gabbros to the volcanic rocks may suggest a similar source and tectonic setting. Thus, the B1 gabbros may be related to rifting and emplacement at the eastern margin of the West African Craton (WAC). The effect of minimal crustal contamination and the arc nature of the B2 gabbros may suggest subduction related magmatism. Taken together, the occurrence of the gabbros may be related to rifting and subduction-collision at the eastern margin of the WAC. © 2016 Elsevier Ltd