Polyphase metamorphism and the development of the Main Central Thrust

Abstract

ABSTRACT Along a cross‐section through the Lesser and Higher Himalayan units at the Kishtwar window area (north‐west India), a polyphase, Barrovian‐type metamorphism has been delineated in relation to the development of the Main Central Thrust (MCT). In the metapelitic mineral assemblages, three metamorphic phases have been distinguished: conditions up to amphibolite grade at moderate to high pressures (alm + rut + ilm + kya + qtz) characterize the M1 phase; pressure release and/or temperature increase as a result of movement along the MCT and the formation of gneiss domes in the Higher Himalaya, as expressed by oriented (N70°‐100° E) fibrolite, defines the M2 phase; and, finally during uplift of the Kishtwar window area, a retrogressive M3 phase is characterized by the assemblage quartz‐muscovite‐chlorite. Both optically zoned and single‐stage garnets have been examined with the electron microprobe to determine their element partitioning. Normal zoning has been found in samples below the MCT in the Lesser Himalaya, indicating prograde growth during the M2 phase, whereas tectonically above, in the Higher Himalaya unit, the garnets reveal double‐stage growth with a complex zoning pattern due to reaction‐partitioning during M1 and M2 and reverse‐zoning at their rims during the retrogressive M3 phase. Geothermometry on metapelites along a cross‐section through the MCT zone and the Higher Himalaya imply distinct readjustments of garnet‐biotite exchange equilibria and indicate isothermal conditions (500‐600° C) throughout the section during the M3 retrogression. Pressure calculations (gro‐an‐kya‐qtz and alm‐rut‐ilm‐kya‐qtz) suggest a decrease in pressure towards the top of the section (6‐7.5 to 4.5‐5 kbar), as corroborated by fibrolite replacing kyanite. The spatially inverse metamorphism exposed within the Lesser Himalaya of the Kishtwar window is regarded as a product of polyphase metamorphism combined with ongoing thrusting and shearing and is reflected by condensed M2 isograds around the Kishtwar window.