Ultra-fast thermal pulses during subduction initiation recorded in metamorphic sole rocks of the Nagaland–Manipur ophiolite Belt

The processes governing subduction initiation remain poorly understood due to the scarcity of geological records capturing its earliest stages. One approach to address this challenge is to study metamorphic sole rocks, which form during subduction initiation. Here we report the pressure-temperature-time (P-T-t) evolution of exceptionally well-preserved metamafic sole rocks, in direct contact with the overlying mantle peridotites from the Nagaland-Manipur Ophiolite Belt (NMOB), India, to elucidate the thermo-tectonic conditions operating during nascent subduction. An integrated approach combining analysis of reaction-textures, thermobarometry, phase equilibria modelling, and diffusion chronometry reveals that: (1) the NMOB sole rocks experienced ultra-high temperature (UHT) metamorphism, reaching 900-960 ℃ at depths of 40-50 km (~1.2-1.5 GPa); (2) the metamorphic evolution was pulsed, marked by two distinct cycles (M₁ and M₂), both along clockwise P-T paths, with repeated heating-cooling at mantle depths; and (3) these events were extremely short-lived, with timescales of 9-40 kyr for the M₁ cooling episode and 17 kyr for the entire M₂ cycle. These results document, for the first time, ultra-fast thermal cycling during subduction initiation. We propose that asthenosphere upwelling, triggered due to the onset of spontaneous subduction of an oceanic plate, along with the resulting processes of mantle-melting and melt migration, provided repeated thermal pulses that caused UHT to high-temperature metamorphism of the NMOB sole rocks. Thus, these findings provide direct geological evidence for the extreme thermal events and rapid tectono-metamorphic feedbacks that can characterize the first ~ 10 s of kyr of subduction on modern Earth.