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.
