Characterizing anatexis in the Greater Himalayan Sequence (Kumaun, NW India) in terms of pressure, temperature, time and deformation

Abstract

We applied field observations combined with P-T pseudosection modelling, zircon U-Pb geochronology and bulk rock geochemistry along the Kali River Valley, Kumaun Himalaya to understand conditions of peak metamorphism and partial melting of the Greater Himalayan Sequence (GHS) along with spatiotemporal relationship between anatexis and fault activation. The southern tectonic boundary of GHS or the Main Central Thrust (MCT) is marked on the basis of structural, metamorphic and chronological evidences. Outcrop-scale observations suggest generation of partial melt at the base of the MCT. This partial melt migrated to higher structural levels and finally emplaced as tourmaline bearing leucogranite in the northern tectonic boundary of the GHS, which is marked by the South Tibetan Detachment Zone (STDZ). P-T pseudosection modelling shows that GHS have experienced muscovite dehydration melting at 9.2–9.8 kbar and 720°–725 °C, 8.4–8.7 kbar and 700°-710 °C, 7.8–8.4 kbar and 700–720 °C respectively at its lower, lower-middle and middle structural levels. Zircon U-Pb geochronology suggests that the GHS underwent suprasolidus peak metamorphism and post-peak anatexis during a time span of ~26–22 Ma at the base of the MCT and ~32–27 Ma at the middle structural level. The MCT is at least ~22 Ma old, being synkinematic to the partial melting event that took place at its base. Diachronous and brief episodes of partial melting and absence of sillimanite zone at the base of the GHS help us envisage a ‘critical taper wedge’ scenario, where partial melt weakened the overlying Himalayan wedge and triggered gravity collapse that formed the STDZ.