Magmatic evolution of the suqii-wagga garnet-bearing two-mica granite, wallagga area, western Ethiopia

Abstract

The Suqii-Wagga two-mica granite, situated in the western Ethiopian Precambrian, is emplaced in a high-grade migmatitic terrane. It is composed of feldspars + quartz + muscovite + biotite ± garnet + zircon ± allanite ± apatite + Fe-Ti oxides + Fe sulphide. Textural studies and microprobe analyses revealed two generations of almandine-spessartine-rich magmatic garnet. The first is euhedral, fine-grained (300–350 μm), commonly occurs as inclusions in plagioclase and alkali feldspars, and exhibits chemical zoning with almandine-rich cores and spessartine-rich rims. In contrast, the second variety is medium- to coarse-grained (1–7 mm) and shows reverse zoning with spessartine-rich cores and almandine-rich rims. Primary and secondary muscovites were discriminated based on the concentrations of Ti, Fe, Mn and Na. Biotite is characterised by a higher alumina saturation index than biotites of other granitoids in the area, suggesting considerable alumina concentration in the source magma. Garnet-biotite thermometry and phengite barometry were used to estimate the P-T conditions of crystallisation for the Suqii-Wagga two-mica granite pluton at ∼ 7 kbar and ∼ 670°C. Mineral paragenesis, the composition of aluminous minerals and the P-T conditions of crystallisation indicate that samples containing fine-grained garnet crystallised earlier than those containing medium- to coarse-grained garnet. Field and petrographic investigations, mineral chemistry, and whole rock major and trace element studies suggest that the Suqii-Wagga two-mica granite has the characteristics of anatectic granite. Highly variable normative Ab/Or ratios suggested melting under varying a H 2O conditions and/or source characteristics. The relatively high Rb/Sr, Rb/Ba and low CaO/Na 2O (<0.3) ratios indicate the derivation of the granitic magma from a plagioclase-poor pelitic source. Moreover, pronounced negative Eu anomalies and large ion lithophile element modelling suggested crystal fractionation involving plagioclase. The presence of the Suqii-Wagga Granite Pluton implies a significant contribution of older mature crustal material to the magmatic evolution of the area.