940 Ma Anatexis in 1726 Ma Orthogneiss in the Northern Margin of the Bhilwara Belt and Significance for the Precambrian Evolution in Northwest India

Abstract

The northwestern domains of India record Proterozoic orogenies that reflect global cycles of convergence and extension. A garnet-biotite migmatitic orthogneiss hosted within the Agucha-Kekri Shear Zone sandwiched between the Bhilwara Belt and the North Delhi Fold Belt (NDFB) has two zircon populations yielding U-Pb ages of 1726 and 938 Ma. The older age is correlated with the intrusion of the migmatite protolith, consistent with the partial melting event recorded in gneisses in the southern margin of the Bhilwara Belt. The younger age is interpreted as the age of partial melting and migmatization. Petrographical observations and pressure-temperature (PT) pseudosection analyses indicate incongruent melting of biotite and plagioclase in the gneiss-produced garnet, potash feldspar, and melt under water-fluxed conditions. The peak conditions of ~9 kbar and ≥700°C estimated for the partial melting are similar to those of coeval migmatization recorded at the northwestern margin of the Bhilwara Belt, but lower than those in the adjacent NDFB. This is interpreted to indicate formation of a migmatitic front along the northwestern margin of the Bhilwara Belt while this was being underthrust under the NDFB. Migmatization under similar PT conditions and, at the same time, estimated for the Central Indian Tectonic Zone implies the presence of several loci of crustal amalgamation leading to the final architecture of peninsular India during Rodinia formation.