Abstract

Coesite, recognized petrographically and confirmed by in situ Raman microprobe spectroscopy, is reported from an eclogite from the Kaghan valley, Pakistan, and represents the first record of ultrahigh-pressure metamorphism in the Himalaya. The formation conditions of >27 kbar implied by the presence of coesite are supported by garnet-pyroxene- phengite barometry (27–29 kbar, 690–750 °C). If, as seems likely from previous field and geochronologic 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 leading edge of the Indian plate— must also have been subducted to coesite-forming depths (90–100 km). This more than doubles previous depth estimates and, on the basis of available geochronological data for this area, requires average exhumation rates at least twice as fast (∼10 mm/yr) as previously imagined. A further implication, based on interpretations of deep seismic data, is that the present-day shallow angle of subduction of Indian plate lithosphere beneath Tibet represents a significant change from an initially much steeper angle.

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