Metamorphic evolution of rocks from the Rajasthan craton, Nw Indian shield

Abstract

This paper describes the metamorphic characteristics of rocks of the Banded Gneissic Complex (3500 to 2600 Ma) and of the supracrustals of the Aravalli (ca. 2000 Ma) and Delhi (1650 to 950 Ma) Supergroups, constituting the Rajasthan Craton and Aravalli mobile belt in the NW Indian Shield. Two regional recrystallization events, separated in space and time, are recorded in the complex, whereas only one regional metamorphism (excluding thermal imprints) is documented in the Proterozoic supracrustals. Sand Mata granulites, a possible component of the Basement Complex and having a shear contact with it, reveal a tectonic path which contrasts with the 'normal' P-T path of most metamorphic terrains and collision zones. In view of the fact that gneissic rocks as old as 3.5 Ga. constitute the Banded Gneissic Complex (BGC) and from the consideration that the Aravalli sequence of the Precambrian gneissic rocks in the NW Indian Shield is ensialic, the present author proposes a model of ensialic orogenesis, instead of the plate tectonic model, for the evolution of the Aravalli fold belt. In this model, the development of large basins (characteristic of the Aravalli-Delhi sediments) is visualized as having formed as a result of ductile extension of hot sialic material, rather than by brittle rupture. Ductile spreading followed by contraction of crust, rather than subduction or plate collision, is considered the dominant process in the evolution of this Proterozoic belt. This large-scale ensialic orogenesis in the NW Indian Shield involved the re-working of the granite gneiss complex, whereby deformed cover rocks are now seen to rest on a mobilized basement. However, there are areas, for example the Berach and Sarara, where deformed cover rocks lie on unmobilized or less mobilized basement.