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

Abstract

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 D 1 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 D 2 folds have a ramp–flat geometry and are refolded by coaxial D 3 folds. The petrology of the metapelites indicates that porphyroblasts of staurolite and/or garnet were formed as a function of bulk-rock composition between D 1 and D 3 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.