Pressure-temperature-time-deformation path of kyanite-bearing migmatitic paragneiss in the Kali Gandaki valley (Central Nepal): Evidence for Late Eocene-Early Oligocene melting

Kyanite-bearing migmatitic paragneiss of the lower Greater Himalayan Sequence (GHS) in the Kali Gandaki transect (Central Himalaya) was investigated. In such paragneisses, Carosi et al. (2014) found crystallized melt inclusions (“nanogranite”) in garnet. These inclusions suggest the presence of a “high-Ca” melt. In spite of the intense shearing, it was still possible to obtain fundamental information for the understanding of processes active during orogenesis. Using a multidisciplinary approach, including meso- and microstructural observations, pseudosection modelling (with PERPLE_X), trace element thermobarometry (e.g. Zr-in-rutile) and in situ monazite U-Th-Pb geochronology, we constrained the pressure-temperature-time-deformation (P-T-t-D) path of the studied rock, located in a structural key position. The migmatitic gneiss has experienced protracted prograde metamorphism after the India-Asia collision (50-55 Ma) from ~ 43 Ma to 28 Ma. During the late phase (36-28 Ma), the gneiss underwent high-pressure melting at “near peak” conditions (710-720 °C/1.0-1.1 GPa) leading to kyanite-bearing leucosomes. In the time span of 25-18 Ma, the rock experienced decompression and cooling associated with pervasive shearing reaching P-T conditions of 650-670°C and 0.7-0.8 GPa, near the sillimanite-kyanite transition. This time span is somewhat older than previously reported for this event in the study area. Taking the migmatitic gneiss as representative of the GHS, these data demonstrate that this unit underwent crustal melting at depths of 35 km (~ 1 GPa) in the Eocene-Early Oligocene, clearly before the widely accepted Miocene decompressional melting related to its extrusion. In general, kyanite-bearing migmatite, as reported here, could be potentially linked to the production of “high-Ca” granitic melts found along the Himalayan belt.