In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry

Abstract

Recent identification of tectono-metamorphic discontinuities within the Himalayan metamorphic core has challenged previous understanding of mountain-building processes/models during continental collision. However, their exact position, spatial extent, and temporal evolution still remain in debate. Monazite petrochronology and thermobarometry have been applied to metapelites of the Gyirong-Syabrubensi-Langtang transect in central Nepal. Peak metamorphic temperatures and pressures were determined by conventional thermobarometry and Zr-in Rutile thermometry. P-T conditions increase structurally upward from 540–580 °C, 8–10 kbar at LHS to 640–710 °C, 8–12 kbar at lower GHC, and 680–720 °C, 6–9 kbar at upper GHC. Petrochronology studies indicate prograde metamorphism in the LHS, partial melting of 20–18 Ma in the lower GHC and prolonged partial melting at 29–23 Ma in the upper GHC, demonstrating a diachronism across these units. The Rasuwagadhi Shear Zone (RSZ), is marked by a jump in T/depth and peak metamorphic timing from lower GHC (<25 °C/km, ~19 Ma) to upper GHC (>30 °C/km, ~29 Ma), and can be correlated with the regional High Himalayan Discontinuity recently identified within the GHC in other Himalayan transects. This shear zone acted almost synchronously with the South Tibetan Detachment around 24–18 Ma and was relayed by movement along the Main Central Thrust after 18 Ma, presenting south-ward in-sequence thrust propagation style. Therefore, a hybrid model, In-sequence Buoyancy Extrusion that involves a combination of underplating and buoyancy mechanisms is proposed to explain the overall exhumation of HMC, and perhaps is applicable to metamorphic core in similar large and hot collisional orogen.