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Reactivation of basement: example from the Anasagar Granite Gneiss Complex, Rajasthan, western India

By
N. Chattopadhyay
N. Chattopadhyay
1
Department of Geological Sciences, Jadavpur University, Kolkata 700032, India
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D. Mukhopadhyay
D. Mukhopadhyay
2
Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700019, India
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P. Sengupta
P. Sengupta
1
Department of Geological Sciences, Jadavpur University, Kolkata 700032, India
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Published:
January 01, 2012

Abstract

Amidst the Meso- to Neoproterozoic South Delhi Fold Belt (SDFB) of Rajasthan, India, a sheet-like body of megacrystic Anasagar Granite Gneiss (AGG) embedded in a supracrustal unit consisting of metapelites, quartzite and calc gneiss is exposed. Detailed analyses of mesoscopic and microscopic structures identify four phases of deformation. Lithological relationships coupled with U–Pb dates of zircon indicate that the protolith of the AGG was emplaced within the supracrustal unit during D1 folding at approximately 1.85 Ga. This event is significantly older than the age of volcanism in the SDFB (0.99 Ga) but probably synchronous with Aravalli Orogeny. Thrusts associated with the easterly-vergent D2 folds have a ramp–flat geometry and are refolded by coaxial D3 folds. The petrology of the metapelites indicates that porphyroblasts of staurolite and/or garnet were formed as a function of bulk-rock composition between D1 and D3 folding, at the time of the culmination of metamorphism (5.7±1.5 kbar, 560±50 °C). Combining the petrological and structural attributes, it is proposed that the AGG and its enveloping supracrustals might represent the basement of the Delhi Supergroup, which was folded, thrusted and domed up during the South Delhi Orogeny. The cause of the thermal perturbation that triggered the growth of the porphyroblasts in the metapelites is explored.

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Geological Society, London, Special Publications

Palaeoproterozoic of India

R. Mazumder
R. Mazumder
Geological Studies Unit, Indian Statistical Institute, Kolkata, India
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D. Saha
D. Saha
Geological Studies Unit, Indian Statistical Institute, Kolkata, India
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Geological Society of London
Volume
365
ISBN electronic:
9781862396135
Publication date:
January 01, 2012

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