Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia

The Suqii-Wagga two-mica granite, situated in the western Ethiopian Precamb rian, is emplaced in a high-grade migmatitic terrane. It is composed of fel dspars + quartz + muscovite + biotite +/- garnet + zircon +/- allanite +/- apatite + Fe-Ti oxides + Fe sulphide. Textural studies and microprobe analy ses revealed two generations of almandine-spessartine-rich magmatic garnet. The first is euhedral, fine-grained (300-350,um), commonly occurs as inclu sions in plagioclase and alkali feldspars, and exhibits chemical zoning wit h almandine-rich cores and spessartine-rich rims. In contrast, the second v ariety is medium- to coarse-grained (1-7 mm) and shows reverse zoning with spessartine-rich cores and almandine-rich rims. Primary and secondary musco vites 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 concentra tion 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 plu ton at similar to 7 kbar and similar to 670 degreesC. Mineral paragenesis, the composition of aluminous minerals and the P-T conditions of crystallisa tion indicate that samples containing fine-grained garnet crystallised earl ier than those containing medium- to coarse-grained garnet. Field and petro graphic investigations, mineral chemistry, and whole rock major and trace e lement studies suggest that the Suqii-Wagga two-mica granite has the charac teristics of anatectic granite. Highly variable normative Ab/Or ratios sugg ested melting under varying a(H2O) conditions and/or source characteristics . The relatively high Rb/Sr, Rb/Ba and low CaO/Na2O (< 0.3) ratios indicate the derivation of the granitic magma from a plagioclase-poor pelitic sourc e. Moreover, pronounced negative Eu anomalies and large ion lithophile elem ent modelling suggested crystal fractionation involving plagioclase. The pr esence of the Suqii-Wagga Granite Pluton implies a significant contribution of older mature crustal material to the magmatic evolution of the area. (C ) 2001 Elsevier Science Limited. All rights reserved.