Geochemistry Of Podiform Chromitite And Host Rocks Within The Buem Structural Unit, Northwestern Benin Republic: Implications For The Geodynamic Evolution Of The Dahomeyide Orogenic Belt.

Abstract

The paper presents the geochemistry, origin and geodynamic setting of podiform chromitite and host rocks from the Buem Structural Unit (BSU) in the northwestern Benin Republic. Qualitative Scanning Electron Microscopy (SEM), quantitative Electron probe Micro Analysis (EPMA), and whole-rock geochemical investigation have been conducted on BSU chromite and mafic-ultramafic rocks. The BSU is the westernmost unit of the Dahomeyide orogenic belt. It consists mainly of metamorphosed mafic-ultramafic rock and clastic sedimentary rocks. Chromitites units are exposed within the mafic-ultramafic rocks. Qualitative SEM analyses indicated that chromitite is mainly composed of chromite and serpentine minerals. The qualitative EPMA data shows that Cr2O3 and MnO contents (wt%) of spinel of the BSU varying from 35.91 to 44.53 and from 0.05 to 0.16, respectively and are typical primary Cr-spinel. The calculated parental composition of melts (13.71–16.67 wt% Al2O3 and FeO/MgO values in the range 0.15–0.30) from which the BSU chromitites precipitated is very similar to MORB-type magma. The chemical composition of BSU chromites and its host rocks shows that these chromites were most likely crystallized from MORB. The magma originated as a partial melt that occurred in a Mid-Ocean Ridge environment, probably during the early stage of the Pan-African orogeny at the margin of the West African Craton.