• Metamorphic sole rocks from the NMOB, NE India are used to study subduction initiation (SI). • Sole rocks document pulsating UHT-HT metamorphism (M 1 -M 2 metamorphic cycles) at mantle depths. • Metamorphic pulses were extremely short-lived, with kyr timescales for the combined M 1 -M 2 cycles. • Extreme thermal events and rapid tectono-metamorphic feedbacks indicate SI at its very infancy. 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 °C at depths of 40–45 km (∼1.2–1.5 GPa); (2) the metamorphic evolution was pulsed, marked by two distinct cycles ( M 1 and M 2 ), 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 M 1 cooling episode and 17 kyr for the entire M 2 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.
Pradhan et al. (Wed,) studied this question.