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

Published online by Cambridge University Press:  30 October 2017

MORGAN L. BLADES ,
JOHN FODEN ,
ALAN S. COLLINS ,
TADESSE ALEMU  and
GIRMA WOLDETINSAE
MORGAN L. BLADES*
Affiliation:
Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia
JOHN FODEN
Affiliation:
Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia
ALAN S. COLLINS
Affiliation:
Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia
TADESSE ALEMU
Affiliation:
Mining Engineering Department, Unity University, P.O. Box 6722, Addis Ababa, Ethiopia
GIRMA WOLDETINSAE
Affiliation:
Research and Development Directorate, Ministry of Mines, Petroleum and Natural Gas, P.O. Box 486, Addis Ababa, Ethiopia
*
†Author for correspondence: [email protected]
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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 fO2 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.

Keywords

Western Ethiopian Shieldchrome spinelAlaskan-type intrusionophiolite
Type
Original Articles
Information
Geological Magazine , Volume 156 , Issue 1 , January 2019 , pp. 62 - 82
Copyright
Copyright © Cambridge University Press 2017 

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