Burial of thermally perturbed Lesser Himalayan mid-crust: Evidence from petrochemistry and P–T estimation of the western Arunachal Himalaya, India

Abstract

In this work, we establish a dual prograde P–T path of the Lesser Himalayan Sequence (LHS) rocks from the Western Arunachal Himalaya (WAH). The investigated metagranites, garnet- and kyanite-zone metapelites of the LHS are part of an Inverted Metamorphic Sequence (IMS) that is exposed on the footwall side of the Main Central Thrust (MCT). Integrated petrographic, mineral chemistry, geothermobarometric (conventional and isopleth intersection methods) and P–T pseudosection modeling studies reveal a near isobaric (at P ∼ 8–9 kbar) peak Barrovian metamorphism with increase in T_Max from ∼ 560°C in the metagranite through ∼ 590–600 °C in the lower and middle garnet-zone to ∼ 600–630 °C in the upper garnet- and kyanite-zone rocks. The metamorphic sequence of the LHS additionally records a pre-Barrovian near isobaric thermal gradient in the mid crust (at ∼ 6 kbar) from ∼ 515°C (in the middle garnet zone) to ∼ 560–580 °C (in the upper garnet- and kyanite zone, adjoining the Main Central Thrust). Further burial (along steep dP/dT gradient) to a uniform depth corresponding to ∼ 8–9 kbar and prograde heating of the differentially heated LHS rocks led to the formation of near isobaric metamorphic field gradient in the Barrovian metamorphic zones of the WAH. A combined critical taper and channel flow model is presented to explain the inverted metamorphic zonation of the rocks of the WAH.