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Structural evolution of migmatites in granulite facies terrane: Precambrian crystalline complex of Angul, Orissa, India

ABSTRACT Basic granulites, a variety of gneisses including sillimanite-garnet gneiss (khondalite), charnockite and intruded quartzofeldspathic material make up a migmatite complex showing evidence of polyphase deformation, polymetamorphism and successive neosome emplacement. The heterogeneity of the migmatites is dominantly the result of folding and boudinage rather than igneous activity. Tight to isoclinal folds of the second recognised deformational phase affect lithological layering, granulite facies fabric elements of the first deformational phase and early neosome; they played a major role in the development of the macroscopically heterogeneous nature of the complex and they are also a key structure for correlation. Upright folds of the third deformational phase control the major disposition of lithological units and, together with their axial planar fabric, controlled the uprise of quartzofeldspathic neosome and of volatiles and heat which caused localised ‘charnockitisation’ of sillimanite-bearing gneisses. The effects of semibrittle and brittle deformation, including kink bands, fractures and shears, express late deformational phases during which there was neosome emplacement, some at 854 ± 6 m.y. ago (Rb–Sr muscovite age).