Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the Neo-Tethys

Abstract

Eclogite from two locations in a fossil accretionary complex now exposed in Nagaland, NE India, at the northern end of the Indo-Myanmar ranges, provides the oldest evidence for Neo-Tethyan subduction along the Yarlung-Tsangpo suture zone. Metamorphic texture, mineral composition, peak metamorphic P-T estimates, reconstructed metamorphic P-T paths, and U–Pb zircon ages from three eclogite samples collected near Mokie and Thewati villages collectively reveal snapshots of a c. 205–172 Ma subduction burial-exhumation cycle in the eastern Neo-Tethys. The Thewati eclogite records a clockwise metamorphic P-T path involving: (1) prograde metamorphism traversing the epidote blueschist facies and culminated in peak eclogite facies conditions at 25–28 kbar and ~650 °C; (2) an early retrograde stage involving decompression with cooling in the eclogite facies at ~18.3 kbar and 630 °C; followed by (3) cooling through the epidote blueschist, transitional lawsonite blueschist and greenschist facies to ~6 kbar and 300 °C. Circa 189–185 Ma peak metamorphism is inferred for the Mokie and Thewati eclogites, and was accompanied by an apparently low thermal gradient (~7–8 °C/km). Such conditions and the clockwise P-T path are associated with a tectonic scenario involving cold mature subduction within the Neo-Tethys in Nagaland. This is thus the first comprehensive dataset of an Early Jurassic subduction channel (with respect to present geographic co-ordinates) for the eastern margin of the Neo-Tethyan suture. The Nagaland samples offer a critical record of a pre-Cretaceous subduction system within the Neo-Tethys, which incorporated an Early-Middle Jurassic Andean-type convergent margin extending along the southern Eurasian margin from Pakistan to the Indo-Myanmar Ranges.