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 Eclogitefrom two locations in a fossil accretionary complexnow exposed in Nagaland, NE India, at the northern end of the Indo-Myanmar ranges, provides the oldest evidence for Neo-Tethyan subductionalong the Yarlung-Tsangpo suture zone. Metamorphictexture, mineral composition, peak metamorphicP-T estimates, reconstructed metamorphicP-T paths, and U–Pb zircon ages from three eclogitesamples collected near Mokie and Thewati villages collectively reveal snapshots of a c. 205–172 Ma subductionburial-exhumation cycle in the eastern Neo-Tethys. The Thewati eclogiterecords a clockwise metamorphicP-T path involving: (1) prograde metamorphismtraversing the epidote blueschist faciesand culminated in peak eclogite faciesconditions at 25–28 kbar and ~650 °C; (2) an early retrograde stage involving decompression with cooling in the eclogite faciesat ~18.3 kbar and 630 °C; followed by (3) cooling through the epidote blueschist, transitional lawsonite blueschistand greenschist faciesto ~6 kbar and 300 °C. Circa 189–185 Ma peak metamorphismis inferred for the Mokie and Thewati eclogites, and was accompanied by an apparently low thermal gradient (~7–8 °C/km). Such conditions and the clockwiseP-T path are associated with a tectonic scenario involving cold mature subductionwithin the Neo-Tethys in Nagaland. This is thus the first comprehensive dataset of an Early Jurassic subductionchannel (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 subductionsystem 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.