Geochemistry of Kolme granitoids: Implication for crustal growth in the Mozambique orogenic belt of southwestern Ethiopia

Abstract

Petrological and geochemical (major and trace element) data are presented for Kolme granitoid from the Precambrian terrain of southwestern Ethiopia. Integrated field and petrochemical studies have been carried out to assess the source and discriminate tectonic setting of the granitoid as well as evaluate their implication for crustal growth. The petrological and the geochemical data indicate the presence of two groups of granitoid in the area: Borkara granitoid (BG) and Matera granitoid (MG). BG is granite in composition with higher SiO2 content (75.27–78.51 wt %), lower Al2O3 (13.05–13.45 wt %), and FeO*/(FeO*+MgO) ꞊ 0.92–0.94 wt %. BG is a high-K calc-alkaline, peraluminous, ferroan, and felsic I-type granitoid. Whereas MG is quartz-monzonite in composition with lower SiO2 (66.03–69.16wt %) and higher Al2O3 (14.85–16.25 wt %) and FeO*/(FeO*+MgO) (0.96–0.97wt %). MG is shoshonitic, metaluminous to peraluminous, alkalic, ferroan and mafic I-type granitoid. BG is derived from the partial melting of upper continental crust with the contribution of melts from subducted slab and asthenospheric mantle wedge, while MG is derived by fractional crystallization of mantle-derived mafic magma underplated in the lower crust with minor crustal involvement.The tectonic discrimination diagrams for Kolme granitoid infer that BG is a syn-orogenic granitoid, while the MG is a post-orogenic granitoid. Moreover, the transition from syn-orogenic calcic-alkalic BG magmatism to alkalic MG magmatism is marking the end of the syn-collisional stage and the beginning of the extension of the Neoproterozoic crust.