The origin of the ultramafic rocks of the Tulu Dimtu Belt, western Ethiopia - do they represent remnants of the Mozambique Ocean?

Abstract

The East African Orogen contains a series of high-strain zones that formed as Gondwana amalgamated. The Tulu Dimtu shear belt is one of these N–S structures within the Barka–Tulu Dimtu zone in western Ethiopia, and contains ultramafic bodies of equivocal origin. Identifying the petrogenetic origin of these enigmatic rocks provides evidence for the geodynamic significance of these shear zones. Owing to their altered state, these ultramafic rocks' well-preserved chrome spinels provide the only reliable evidence for their source and tectonic affiliation. Chrome spinels have high Cr₂O₃ (30.04–68.76 wt %), while recalculated Fe₂O₃ (<2%) and TiO₂ (0.01–0.51%) values are low. The Cr# (molar Cr³⁺/Cr³⁺ + Al²⁺) and Mg# (Mg²⁺/Mg²⁺ + Fe²⁺) have averages of 0.88 and 0.22, respectively. Based on olivine–spinel equilibria, the calculated ƒO₂ values (FMQ +3.03) for the dunites reveal a highly oxidized environment. This spinel chemistry (high Cr# > 0.6 and low Ti) supports a supra-subduction origin, with an oxidized mantle source more refractory than depleted MORB mantle (DMM). These spinel compositions indicate that some ultramafic bodies in western Ethiopia, including those from Daleti, Tulu and Dimtu, are serpentinized peridotites emplaced as obducted ophiolite complexes. By contrast, the ultramafic rocks from the Yubdo locality have a different spinel chemistry, with strong affiliation with igneous spinels formed in Alaskan-style mafic intrusions. These collective results suggest that regardless of their origin as supra-subduction ophiolites or as Alaskan-type intrusions, these spinels were formed on a convergent-subduction margin.