A polyphase evolution and a thermal and mineral zonation characterize metamorphism of the High Himalayan Crystallines (HHC) in the Everest transect. After an early Barrovian-type metamorphism (M1: T = 550–680° C; P = 8–10 kbar) caused by subduction and crustal thickening during India-Eurasia collision, exhumation of the HHC occurred at rates which allowed the isotherms to rise and the thickened crust to suffer partial melting. In the upper structural levels, the thermal conductivity contrast between crystalline and upper sedimentary rocks together with the infiltration of metamorphic fluids released during exhumation, favoured migmatization and formation of leucogranite magmas through partial melting of muscovite-bearing metasediments chiefly at medium (M2: T = 650–750° C; P = 4–7 kbar) but also at low-pressure (M3: T = 600–700° C; P = 2–4 kbar). The leucogranite melts segregated as dykes and kilometre-sized bodies into extensional shear zones situated at the uppermost levels of the HHC and at the base of the overlying Tibetan Series. During exhumation, a metamorphic zonation was imposed on the HHC in response to the complex interplay between rise of the isotherms in the thickened crust, emplacement of leucogranites in the uppermost levels of the HHC and conductive cooling near the ‘cold’ Lesser Himalaya and Tibetan Series coupled with thrust imbrication.

-In the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as Ml and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle" structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb-Sr and U-Th-Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the 'cooler' Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the latemetamorphic Zanskar Fault.

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The High Himalayan Crystallines (HHC) from the little-known NE termination of the Nanga Parbat-Haramosh syntaxis (Stak and upper Turmik valleys, Pakistan) consist of kyanite-bearing gneiss with minor garnet-granulite and garnet-amphibolite. The HHC underwent a Himalayan metamorphism with a peak at high pressure (8–13 kbar) and high temperature (650–700° C). During exhumation the HHC rocks followed a rapid exhumation path at high temperature with little or no medium to low pressure re-equilibration. These lines of evidence, combined with geochronological and petrological data from Eocene eclogites recently found in the Kaghan nappe, indicate that, after subduction and high to very high pressure metamorphism, part of the HHC from northern Pakistan underwent very rapid cooling and exhumation. By contrast, exhumation along paths of increasing temperature are recorded by the HHC from regions located east of Pakistan (e.g. Nepal). These differences along strike in the HHC suggest that, during the Eocene collision between India and Eurasia, subduction and exhumation occurred at higher rates in northwestern than in central-eastern Himalaya.

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