Timing of Himalayan ultrahigh-pressure metamorphism: sinking rate and subduction angle of the Indian continental crust beneath Asia

Coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from Higher Himalayan Crystalline rocks in the upper Kaghan Valley, north-west Himalaya. The metamorphic peak conditions of the coesite-bearing eclogites are estimated to be 27-32 kbar and 700-770 degreesC, using garnet-pyroxene-phengite geobarometry and garnet-pyroxene geothermometry, respectively. Cathodoluminescence (CL) and backscattered electron (BSE) imaging distinguished three different domains in zircon: inner detrital core, widely spaced euhedral oscillatory zones, and thin, broadly zoned outermost rims. Each zircon domain contains a characteristic suite of micrometre-sized mineral inclusions which were identified by in situ laser Raman microspectroscopy. Core and mantle domains contain quartz, apatite, plagioclase, muscovite and rutile. In contrast, the rim domains contain coesite and minor muscovite. Quartz inclusions were identified in all coesite-bearing zircon grains, but not coexisting with coesite in the same growth domain (rim domain). (206) Pb/U-238 zircon ages reveal that the quartz-bearing mantle domains and the coesite-bearing rim were formed at c. 50 Ma and 46.2 +/- 0.7 Ma, respectively. These facts demonstrate that the continental materials were buried to 100 km within 7-9 Myr after initiation of the India-Asia collision (palaeomagnetic data from the Indian oceanic floor supports an initial India-Asia contact at 55-53 Ma). Combination of the sinking rate of 1.1-1.4 cm year(-1) with Indian plate velocity of 4.5 cm year(-1) suggests that the Indian continent subducted to about 100 km depth at an average subduction angle of 14-19degrees.