THE GEOTECTONIC EVOLUTION OF THE WESTERN ETHIOPIAN SHIELD

Abstract

The western Ethiopian Shield comprises three lithotectonic units. The Birbir domain, an assemblage of mafic to felsic intrusive and extrusive rocks and mainly volcanogenic sedimentary rocks, is enclosed between the dominantly orthogneissic Baro and Geba domains. The earliest recorded deformation event (D1) resulted in the formation of a subhorizontal gneissosity within the gneissic terranes which was synchronous with an early upper amphibolite-facies metamorphic peak (M1) at 800–770 Ma which locally caused partial melting. All terrains were subsequently deformed in the D2 event which was the result of severe E-W crustal shortening. An anticlockwise P-T-t path is implied. Subsequent D3 deformation was concentrated within mylonitised domain boundaries which record major transcurrent movement. These structures were reactivated and suffered fluid incursion resulting in isotopic reequilibration at 635–580 Ma. A second metamorphic event, M2, related to crustal thickening and consequent granite genesis, occurred after the cessation of D3 shearing. Bulk chemical analyses show that the metamorphosed plutonic and volcanic rocks of the Birbir domain are predominantly calc-alkaline and similar to those generated by subduction in modern magmatic arcs. They belong in part to the low-K series, suggesting an oceanic environment. The evolution of the region can be explained in terms of the melting of a subducting slab, intrusion, metamorphism and the formation of an oceanic island arc complex. Continued plate convergence caused severe east-west shortening and basin closure. Further attenuation gave rise to transcurrent shearing, fluid influx, a second thermal event and accretion of microcontinents.