The structural setting of the Chilka Lake granulite-migmatite-anorthosite suite with emphasis on the time relation of charnockites

Abstract

Abstract The granulites and anorthosites occurring around Chilka Lake in Orissa display polyphase deformation structures which have been utilized to constrain the deformation history and the time relation between the rock units. The granulites are represented by metapelitic khondalites and migmatitic charnockite-leptynite complexes. Structural mapping complemented by examination of aerial photographs reveals imprints of three folding episodes. The dominant NE-SW tectonic trend was produced by the first folding F 1 with S 1 preserved in khondalites and charnockites. The second episode of folding was tight, coaxial with F 1 , resulting in hook-shaped folds described on the map scale by the khondalites. In the leptynitic orthopyroxene-free garnetiferous granite gneiss component of the migmatite complex, S 1 is absent, and the major structural grain is due to the gneissose banding S 2 . The third folding affected the metapelites and the migmatite complex differently. In the metapelites the F 3 folds, which are asymmetric, show a uniformly gentle plunge towards the NE/NNE, and the dome-basin patterns ascribed to F 3 by Sarkar et al. (1981) are not discernible. In the migmatite complex these folds, though nearly upright, are disharmonic, probably a consequence of a different rheology owing to the presence of melts. The anorthosites register the imprint of F 3 only and are thus distinctly younger. The retention of S 1 in the charnockite-enderbite layers and lenses within composite charnockite-leptynite exposures, transposition of S 1 to the leptynitic foliation S 2 in several locales, and the different orientation of S 1 in charnockite patches in the different limbs of F 3 folds are strong indications of the charnockites being older than the leptynites. The charnockite “measles” occurring near the hinge regions of the F 3 structures cannot be construed as sites of in-situ arrested charnockitization, instead they are discontinuous relicts of charnockite layers which have been deformed, torn and displaced. We argue that separation of structures of different generations is necessary for a proper understanding of the field features taken to indicate arrested formation of charnockites in South India and elsewhere.