Coesite in Himalayan eclogite and implications for models of India-Asia collision

Coesite, recognized petrographically and confirmed by in situ Raman micropr obe spectroscopy, is reported from an eclogite from the Kaghan valley, Paki stan, and represents the first record of ultrahigh-pressure metamorphism in the Himalaya. The formation conditions of > 27 kbar implied by the presenc e of coesite are supported by garnet-pyroxenephengite barometry (27-29 kbar , 690-750 degreesC). If, as seems likely from previous field and geochronol ogic studies, the eclogites represent metamorphosed dikes, sills, and lava flows of Permian age within the granitic gneiss-metapelite-marble sequence of the Higher Himalayan crystalline nappes, then continental crust-the lead ing edge of the Indian plate-must also have been subducted to coesite-formi ng depths (90-100 km), This more than doubles previous depth estimates and, on the basis of available geochronological data for this area, requires av erage exhumation rates at least twice as fast (similar to 10 mm/yr) as prev iously imagined. A further implication, based on interpretations of deep se ismic data, is that the present-day shallow angle of subduction of Indian p late lithosphere beneath Tibet represents a significant change from an init ially much steeper angle.