Extending along the east coast of peninsular India, the Eastern Ghats expose a deep section through a composite orogenic belt that once formed part of the Proterozoic mobile belt system within East Antarctica and East India. The critical evaluation of the existing geological and isotopic data strongly suggests that this orogenic belt includes not only the granulite facies Eastern Ghats Belt but also the Nellore-Khammam Schist Belt and lower grade units at the southern margin of the Singhbhum Craton. The present authors propose its subdivision into four crustal provinces with widely different geological evolutions. The Rengali and Jeypore Provinces formed at the margin of the Bhandara Craton in the Late-Archaean. In the Krishna Province, volcanosedimentary rocks equivalent to the Cuddapah Supergroup accumulated, probably on the Dharwar Craton in the Palaeoproterozoic, and the major tectonometamorphic event took place between 1.67 and 1.55 Ga, subsequent to a short-lived igneous activity. The Eastern Ghats Province, which shows considerable similarities with the Rayner Province of East Antarctica, was strongly affected by pervasive deformation, high-grade metamorphism and crustal-derived magmatism between 1.1 and 0.9 Ga, which extensively modified the crustal structure of present eastern peninsular India. Neoproterozoic and Early Phanerozoic tectonothermal activities were largely restricted to pre-existing shear zones, but the present configuration of the composite orogenic belt may have been achieved only during the Pan-African Orogeny.
Arrested-type charnockite formation occurs in an assemblage of high-grade gneisses at several localities of the Chilka Lake area that belongs to the Proterozoic Eastern Ghats Belt of India. The isolated ellipsoidal domains are 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 multiple deformation of the rock assemblage under high-grade conditions. The intrusion of the leucogranitic melts separates a first episode of deformation, D 1 , from a younger progressive deformation, D 2 -D 4 . A transpressive regime and inhomogeneous deformation is indicated for D 2 -D 4 by the associated structures and fabrics. But quartz caxis patterns show that pure shear prevailed during the closing stages of deformation. The spatial distribution and orientation of the ellipsoidal charnockite domains within the host leptynite and the orientation pattern of orthopyroxene c-axes inside the domains provide evidence for a synkinematic in situ formation of the domains during D 3 , through partial breakdown of the leptynite assemblage (Bt + Grt + Qtz + Fl 1 8Opx + Fsp + Ilm + Fl 2 /L). Local fluid migration along steep foliation planes associated with large-scale D 3 folds triggered the reaction. Orthopyroxene blastesis was confined to the centre of the domains, and an envelope formed in which the residing fluid caused secondary intergranular formation of chlorite, ore and carbonate, imparting the domains' 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 responsive to the local stress field. Comparison of chemical rock compositions supports the in situ formation of charnockite in leptynite. Subtle compositional differences are controlled by the changing mineralogy. Compared to the host leptynite, the charnockite domains are enriched in K 2 O, Ba, Rb and 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 Bhattacharya, Sen & Acharyya, but formed in situ in the leptynite as a result of localized synkinematic fluid migration late in the deformation history.
Petrological and structural investigations in metapelites and amphibolites were undertaken to unravel the Variscan tectonic and metamorphic evolution of the southwestern Aiguilles Rouges massif. The geometry of planar and linear structures indicates a transpressive deformation regime with a change from predominantly subhorizontal to subvertical movements during its evolution. In metapelite samples from gneiss areas, garnet zonation trends have been correlated with chemical variations in plagioclase and biotite by microstructural observations. Applied conventional cation-exchange and net-transfer geothermobarometers yielded anti-clockwise P-T-deformation paths. Clockwise P-T-deformation paths resulted from (Na, Ca)-amphibole zonations of amphibolite samples from a different lithotectonic unit. Notable differences in P-T conditions and the general shape of P-T paths for the gneisses and amphibolites indicate that the units do not represent a continuous metamorphic zonation and underwent no common tectono-metamorphic evolution throughout the entire deformation. The combination of P-T-deformation paths and structural investigations revealed first-stage thrust tectonics followed by a transpressive deformation which was responsible for the uplift of the southwestern Aiguilles Rouges massif.
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