Subduction initiation in the Neo-Tethys: constraints from counterclockwise P–T paths in amphibolite rocks of the Nagaland Ophiolite Complex, India

Abstract

High-P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along Counterclockwise (CCW) P–T paths. Conceptual and thermo-mechanical models, however, predict multiple burial–exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle-Late Jurassic Neo-Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium-grade amphibolite tectonic unit within a serpentinite mélange, we establish two overprinting metamorphic cycles (M₁–M₂). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M₁ metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende-eclogite facies at 13.8 ± 2.6 kbar, 625 ± 45°C (error 2σ values) and subsequent cooling and partial exhumation to greenschist facies. The M₂ metamorphic cycle reflects epidote blueschist facies prograde re-burial of the partially exhumed M₁ cycle rocks to peak metamorphism at 14.4 ± 2 kbar, 540 ± 35°C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M₁ metamorphism as the first evidence for initiation of subduction of the Neo-Tethys from the eastern segment of the Indus-Tsangpo suture zone. Reburial and final exhumation during M₂ are explained in terms of material transport in a large-scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo-Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.