Intracontinental orogenesis in an ancient continent-continent collision zone: Evidence from structure, metamorphism and P-T paths across a suspected suture zone within the Eastern Ghats Belt, India

摘要

In the Eastern Ghats Belt of India, deformation and granulite facies metamorphism are generally correlated with India-Antarctica collision during the assembly of Rodinia. Within the belt, Archean granulites in the west are separated from Proterozoic granulites to the east by a prominent lithotectonic contact. Recent studies assume this contact to be the suture zone related to collision, although the structural and metamorphic evolution of the Archean granulites are so far undescribed. In this study, an integrated structural and metamorphic study conducted across the contact reveals that each unit preserves unrelated early granulite facies fabrics, but subsequently shared a top-to-the-west thrusting event that imprinted a penetrative NE-SW trending, southeasterly dipping shear foliation along the boundary zone. Syn-thrusting fluid infiltration initially stabilized hydrous minerals at the cost of earlier granulite facies assemblages in both units. Renewed heating and loading then re-established granulite facies conditions in both units, with peak conditions around 8 kb, 750-800 ℃, possibly in the mid-Neoproterozoic time. Since metamorphic conditions and P-T trajectories are similar across the contact, the pre-thrusting thermal structure in both units is inferred to be similar. This implies that both units were already amalgamated into a common footwall block prior to this shared tectonothermal event. No ophiolites, arc magmas or post-collisional granitic activity of Neoproterozoic age are associated with this contact. It is concluded that the contact between Archean and Proterozoic granulites in the Eastern Ghats Belt is not a suture but records intracontinental orogenesis within the collision belt that either predates or post-dates the Proterozoic India-Antarctica amalgamation.