Petrology and geochemistry of the Gore-Gambella plutonic rocks: implications for magma genesis and the tectonic setting of the Pan-African Orogenic Belt of western Ethiopia

Abstract

Major and trace element data for granitoid rocks from the Gore-Gambella Precambrian terranes of western Ethiopia have been used to infer their petrogenetic history and the tectonic environment in which the magmas were emplaced. The plutons are found in the lithotectonic domains of Birbir, Baro and Geba, and were emplaced during the interval between 830 and 540 Ma. Geochemical data show that the Neoproterozoic, pre- to syn-kinematic metaplutonic rocks of the Birbir domain are predominantly calc-alkaline with minor low K trondhjemitic granodiorites and tonalites. Some syn-kinematic granitoids, occurring as sheets or lenticular bodies within high-grade metamorphic rocks of the Baro domain, are peraluminous and show a wide range of incompatible trace elements such as Y and Zr. The post-kinematic plutons are enriched in K and incompatible elements relative to older intrusives, suggesting a shoshonitic petrochemical affinity. Petrological and geochemical data show that the bulk of the pre- to syn-kinematic rocks represent mantle-derived calc-alkaline and arc-tholeiitic magmas. Major and trace element variations suggest a multistage, probably polybaric evolution for these intrusions. The syn-kinematic granitoid lenses of the Baro domain have petrological and geochemical characteristics that suggest a genesis by crustal melting. The late- to post-kinematic plutons are probably related to the melting of calc-alkaline continental crust, followed by fractional crystallisation or AFC. Overall, the plutonic activity reflects the geotectonic evolution of the region through time. The pre- to syn-kinematic rocks from the Birbir domain are compositionally akin to those generated by subduction in modern magmatic arcs; they belong in part to a low K series and are interpreted as emplaced in an intraoceanic island-arc environment. The occurrence of greywacke turbidites and rare carbonates of submarine deposition supports this hypothesis. The late- to post-kinematic intrusions possess geochemical signatures that imply a role for both subduction-related and intraplate components.