β-shaped metamorphic P-T path as a monitor of Mesoarchaean tectonics: new insights from the Coorg Granulite Block, South India

β-shaped metamorphic P-T path, which has emerged as a hallmark of Phanerozoic terrane accretion, may act as a critical proxy to recognise possible convergent plate margin tectonics in the Archaean. However, its efficacy in fingerprinting Archaean terrane accretion is yet to be tested as such paths are currently unrecognised in the Archaean rock record. We report here the first unequivocal β-shaped P-T path from a Mesoarchaean metamorphic belt and discuss its tectonic significance, for which we apply individual thermobarometry, phase equilibria modelling, U-Pb monazite geochronology and garnet-orthopyroxene diffusion chronometry to metasedimentary granulites from Coorg granulite block, South India. These reveal four sequential metamorphic stages: (a) prograde burial to high-pressure-ultra-high temperature peak conditions (∼950–1080°C at 10.4–11.4 kbar); (b) decompression and cooling to ∼7.6–8 kbar, 650–750°C; (c) mid-crustal heating to ∼870°C at ∼7 kbar; and (d) final decompression and cooling to 6–6.4 kbar, 540–650°C. This pulsating metamorphism features two clockwise heating–cooling and burial–exhumation cycles (M 1 and M 2 ; ∼3.16–3.12 Ga), defining a β-shaped P-T path. Grt-Opx diffusion modelling results are compatible with the β-shaped thermal history. While the total duration of the metamorphic pulses range between 12.9 and 14.1 Myrs, the individual pulses are short-lived (e.g. Δt (M 1 ) ∼ 10.7 Myrs; Δt (M 2 ), between 2.2 and 3.4 Myrs). The cooling rates of granulites changed from an early slower (∼ 30°C/Myr) to a later faster (between 100 and 200°C/Myr) one. We attribute this crustal recycling, including the first documented Mesoarchaean late heating to either recurring sagduction tectonics or plausibly to peel-back orogenesis on a hotter Earth. Contrasting P-T paths in the Coorg Granulite Block and Western Dharwar Craton (∼3.16–2.9 Ga) additionally suggest a Mesoarchaean thermal duality, pushing the onset of possible embryonic plate-margin processes back to the Mesoarchaean, predating Proterozoic examples.