Petrogenetic Evolution of the Eastern Buem Volcanic Rocks, South-Eastern Ghana

Abstract

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.