Metamorphic and structural evolution of the Haimanta Group: insights from pressure–temperature–time–deformation path modelling of rocks near the Leo Pargil dome, NW India

The metasedimentary rocks of the Haimanta Group in the upper Sutlej Valley, NW India, record a history of Himalayan-age deformation and metamorphism. Situated beneath the low-grade rocks of the Tethyan sedimentary sequence in the hanging wall of the South Tibetan Detachment System (STDS), these poly-deformed rocks record Eocene–Oligocene burial metamorphism and crustal thickening and shortening, and early Miocene decompression and mid-crustal melting, followed by exhumation along extensional shear zones related to orogen-parallel exhumation of the Leo Pargil dome (LPD). Detailed petrography indicates a gradient in the intensity of the low-pressure overprint in the Haimanta Group that increases with proximity to the LPD, suggesting that exhumation of this portion of the Haimanta Group was mechanically coupled with exhumation of the dome. New U–Th/Pb monazite ages place a maximum limit on the development of the crenulation cleavage (S 2 ) at c . 31 Ma and the development of low-pressure overgrowths on Barrovian porphyroblasts at c . 27 Ma. Quantitative thermobarometry and pressure–temperature–time–deformation ( P – T – t – D ) path modelling suggests that Haimanta Group schists proximal to the LPD record near-isothermal decompression from peak conditions. Combined with previously reported age limits for peak burial, these results indicate that this portion of the Haimanta Group was initially exhumed from mid-crustal depths at a rate similar to the rocks to the SW that do not show a low-pressure overprint on Barrovian assemblages, although their respective retrograde P – T paths show markedly different trajectories. We argue that the transition from moderately sloped to steep retrograde P – T paths within the Haimanta Group is gradational and does not coincide with a tectonic boundary, and it was orogen-parallel extension along the Leo Pargil shear zone that resulted in the differences in the P – T histories observed in these rocks.