Petrological and Geochemical Constraints on the Evolution of the Alkaline Complex of Koraput in the Eastern Ghats Granulite Belt, India

Abstract

The Koraput Alkaline Complex in the high-grade Eastern Ghats Belt, India, is synkinematically emplaced in a pull-apart structure and far from the Bastar cratonic margin. The suite comprises four distinct members, namely, mafic syenite, felsic syenite, nepheline syenite and perthite syenite. Fe-rich orthopyroxene rims on olivine in mafic syenite indicate iron-enrichment in the early stage of differentiation. With plagioclase of andesine composition it could be described as alkali-norite, the plutonic equivalent of hawiite. Felsic syenite with both alkali-feldspars and plagioclase of oligoclase composition could be described as two-feldspar syenite, the plutonic equivalent of mugearite. Albitic rims on nepheline indicates subsolvus reaction. Chemical trends in amphiboles and plagioclase feldspars, progressively more ferroan and more sodic respectively, are strong indications of mineral fractionation in the Koraput suite. Chemical trends in the variation diagrams are compatible with feldspar fractionation in the Koraput suite. A weak Fe-enrichment trend in the early stage of differentiation, as observed in the AFM diagram, is compatible with that of the alkali-basalt series. Nb anomalies, both positive and negative, are indicative of crustal contamination as expected in synkinematic plutons. In terms of Gondwana assembly and break up, the alkaline complexes are supposed to represent rift-related magmatism along the continental margin. In spite of petrological evidence of the magmatic character of the Koraput Complex, anorogenic setting is contra-indicated by mesoscopic and microscopic fabrics, more akin to synkinematic intrusion during F 2 folding in the host country rocks. The Proterozoic alkaline complexes in the Eastern Ghats Belt, could alternatively trace the path of moving Gondwana continent over mantle plumes.