Two Suites Of Gabbros In The Buem Structural Unit, Of The Pan-African Dahomeyide Orogen, Southeastern Ghana: Constraints From New Field And Geochemical Data.

Abstract

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.