On the origin of "arrested' charnockitization in the Chilka Lake area, Eastern Ghats Belt, India: a reappraisal

Arrested-type charnockite formation occurs in an assemblage of high-grade g neisses at several localities of the Chilka Lake area that belongs to the P roterozoic Eastern Ghats Belt of India. The isolated ellipsoidal domains ar e found exclusively in leucogranite (leptynite) bands that intruded litpar- lit interbanded granulite-grade supracrustal and intermediate igneous rocks (khondalite-enderbite). Macrostructures and microfabrics document a multip le deformation of the rock assemblage under high-grade conditions. The intr usion of the leucogranitic melts separates a first episode of deformation, D-1, from a younger progressive deformation, D-2-D-4. A transpressive regim e and inhomogeneous deformation is indicated for D-2-D-4 by the associated structures and fabrics. But quartz c-axis patterns show that pure shear pre vailed during the closing stages of deformation. The spatial distribution a nd orientation of the ellipsoidal charnockite domains within the host lepty nite and the orientation pattern of orthopyroxene c-axes inside the domains provide evidence for a synkinematic in situ formation of the domains durin g D-3, through partial breakdown of the leptynite assemblage (Bt + Grt + Qt z + Fl(1) reversible arrow Opx + Fsp + Ilm + Fl(2)/L). Local fluid migratio n along steep foliation planes associated with large-scale D-3 folds trigge red the reaction. Orthopyroxene blastesis was confined to the centre of the domains, and an envelope formed in which the residing fluid caused seconda ry intergranular formation of chlorite, ore and carbonate, imparting the do mains' typical greenish-brown charnockite colour. The shape of the envelope , which varies from prolate in limbs to oblate in hinges of D-3 folds, is r esponsive to the local stress field. Comparison of chemical rock compositio ns supports the in situ formation of charnockite in leptynite. Subtle compo sitional differences are controlled by the changing mineralogy. Compared to the host leptynite, the charnockite domains are enriched in K2O, Ba, Rb an d Sr, but depleted in FeO*, MnO, Y and Zr. The data obtained in this study provide conclusive evidence that the ellipsoidal charnockite domains do not represent remnants of Stretched enderbite layers as proposed by Bhattachar ya, Sen & Acharyya, but formed in situ in the leptynite as a result of loca lized synkinematic fluid migration late in the deformation history.