Geochemistry of the Neoproterozoic ophiolite from the Bensa-Girja area, southern Ethiopia: Petrogenesis and paleotectonics

Abstract

The Bensa-Girja area is located in northernmost part of the Neoproterozoic Adola belt, southern Ethiopia. It is largely constituted of mafic rocks (amphibolite and metagabbro), ultramafic suite (serpentinite/serpentinized peridotite, talc schist and subordinate pyroxenite), metasedimentary (metapelitic to semi-metapelitic schists, minor marble and metasandstone) and metavolcaniclastic rocks. Geochemical analysis was carried out on the mafic–ultramafic rocks to characterize their petrogenesis, magma source and paleotectonic setting. The results indicate that the mafic rocks have tholeiitic to boninitic composition and represent primitive to evolved magma. The current serpentinite and serpentinized peridotite were formed after harzburgites that are interpreted as a refractory mantle wedge peridotite, and pyroxenites are considered as cumulate rocks crystallized from partial melts of the peridotite. The geochemical data further suggest that the mantle sources of the mafic-ultramafic rocks were modified by both slab-derived fluids and melts in a supra-subduction zone. The rocks thus display geochemical characteristics typical of island arc system with slight MORB-like affinity, inferred to have formed in a sub-arc or spreading forearc setting. It is interpreted that the studied rocks are representing remnants of a fragmented oceanic lithosphere. This together with field geological data supports the arc accretion model postulated for the crustal growth of the Arabian-Nubian Shield.