Inverted metamorphic sequence in metamorphic sole rocks of the Nagaland–Manipur Ophiolite Belt, NE India: metamorphic field gradient and subduction initiation tectonics

This study reports a sequence of high- and medium-grade metamorphic rocks beneath structurally overlying mantle peridotites, referred to as a ‘metamorphic sole’ in the Nagaland–Manipur Ophiolite Belt, NE India. Here, we characterize the petrography and mineral chemistry, constrain the pressure-temperature conditions, and show the sole records an inverted metamorphic gradient. Structurally downwards beneath the deformed mantle peridotite, the sole sequence consists of mafic granulites (peak <i>P–T</i> at ~1.35–1.48 GPa, ~870–950°C) at the top, through garnet-clinopyroxene bearing amphibolite (~1.25–1.29 GPa, ~785°C) and garnetiferous and non-garnetiferous amphibolite (~1.17–1.27 GPa, ~700–720°C) to epidote amphibolite (~1.2–1.33 GPa, ~625–680°C). The studied sole sequence is distinct as it records high-pressure, ultrahigh-temperature granulite facies metamorphism at the tip of the subducted oceanic crust during subduction initiation. The granulite facies metamorphic sole rocks exhibit a progressive decrease in peak metamorphic temperatures from ~950°C to ~870°C at ~1.4 GPa, structurally down the section. The metamorphic sole sequence defines a metamorphic field gradient with either a near-isobaric temperature gradient at mantle depths or a gradient with a gentle, positive slope in the <i>P</i>–<i>T</i> space. These findings, when collated with the results of thermo-mechanical modelling and theoretical considerations, suggest that subduction in the southeastern arm of the Neo-Tethys was most likely initiated through a subduction initiation mechanism, after which the different slices of sole rocks were progressively accreted at either uniform depths under mantle condition or at shallower depths (between 1.5 and 1.2 GPa), as the subduction interface cooled.