U–Pb geochronology and metamorphic history of gneissic rocks from Sarwar‐Junia Fault Zone, Rajasthan, NW India: Implications for the tectonothermal evolution of the Aravalli‐Delhi Mobile Belt

Abstract

The gneisses of the Sarwar‐Junia Fault Zone (SJFZ), in the Mangalwar Gneissic Complex, a part of the Banded Gneissic Complex (BGC‐II) of northwest India recorded two phases of metamorphism. The pelitic gneiss from SJFZ mainly consists of biotite, sillimanite, garnet, quartz, K‐feldspar and plagioclase. On the other hand, the granitic gneiss from SJFZ mainly consists of quartz, plagioclase, K‐feldspar and biotite. From zircon and monazite geochronology, the protolith of pelitic gneiss was deposited on basement granitic gneiss and the metamorphism occurred at ca. 950 Ma affecting both the granitic gneiss and the pelitic gneiss. Estimation of the P‐T conditions of metamorphism and pseudosection modelling of the pelitic gneiss revealed that the peak stage metamorphic condition of M2 culminated at ~850°C and ~7 kbar pressure followed by a near‐isobaric cooling. Large garnet porphyroblasts in contact with retrograde biotite shows Fe‐Mg resetting close to the core region, which implies a slow cooling. Therefore, the possible exhumation during the subduction‐collision tectonics was very slow, if not completely ceased, at ca. 950 Ma. The shared tectonothermal history of the Sandmata Metamorphic Complex and Mangalwar Metamorphic Complex in the BGC‐II is now evident from the present study of the SJFZ, and confirms the presence of Grenvillian‐aged tectonothermal events in the eastern part of BGC‐II. The abovementioned near‐isobarically cooled and hydrated crust of the SJFZ records subsequent event(s) of shallow crustal brittle faulting as evident by the presence of pseudotachylyte veins and cataclastic fabric observed in the studied gneisses. Brittle faulting possibly occurred between ca. 905 Ma and ca. 750 Ma, as interpreted from the present geochronological data.