Geology and geochemistry of the Neoproterozoic Tuludimtu Ophiolite suite,western Ethiopia

The Kemashi Domain, a lithotectonic subdivision of the Neoproterozoic Tuludimtu Orogenic Belt of western Ethiopia, consists of a suite of mafic–ultramafic volcanic and plutonic rocks, and interbedded deep marine sediments, mainly graphite-bearing pelitic schists and phyllites, and graphitic quartzites and cherts. Pillow structures indicate submarine extrusion of the volcanics, whilst partings within some of the basalts may represent sheeted dykes. An associated mélange unit, composed of blocks of the same rock types as above, set in a fine schistose matrix, also occurs. This assemblage is interpreted as a dismembered ophiolite—the Tuludimtu Ophiolite—formed in a deep oceanic environment. A turbiditic sequence is also present in the domain.The Tuludimtu Ophiolite underwent intense compression during the Neoproterozoic Pan African Orogeny, resulting in early recumbent folding and westwards-directed thrusting, followed by reactivation of steeper zones of the thrusts as N–S orogen-parallel strike-slip shear zones, accompanied by refolding of early folds into upright horizontal folds. This was followed by development of deep crustal NNW–SSE orogen-transecting shear zones, which were reactivated as brittle faults during orogenic collapse of the Tuludimtu Belt. Metamorphism to lower greenschist facies grade accompanied orogenesis.Major, trace and REE geochemistry of volcanic and some plutonic igneous rocks, has been employed to define the tectonic setting of the terrane. Tectonic discrimination diagrams, utilising REE and HFSE, indicate a wide distribution spectrum but with the majority of samples plotting in arc basalt and MORB fields, suggesting derivation from sources similar to N-MORB and depleted MORB (typical of many arc basalts). Most of the samples exhibit a slight depletion of immobile elements, relative to N-MORB values and also show depletion of Zr, Ti, Nb and Y, implying that their source had been depleted by an earlier melting episode. Overall, the geochemistry typifies spreading centre basalts with some compositional features transitional to those of arc basalts, a characteristic of back-arc basalts.Lithological association, structural style and geochemistry of the rock assemblage in the Kemashi Domain thus define an ophiolite interpreted to have formed within a deep marine environment. This is thought to have been due to rifting of continental crust within a back-arc basin regime in a continental margin type extensional setting. Comparison with other ophiolitic terranes within the Arabian Nubian Shield, suggests that many of these terranes may represent back-arc basin type tectonic settings, similar to the Kemashi Domain. This supports the multi-stage accretion model for closure of the Mozambique Ocean, for which the Pacific Ocean may be a present day analogue.