NEAR-ULTRAHIGH PRESSURE PROCESSING OF SUBDUCTED CONTINENTAL CRUST: MIOCENE CRUSTAL XENOLITHS FROM THE PAMIRS

Xenoliths erupted at 11 Ma in the southeastern Pamirs suggest that Gondwanan crust overprinted by a Cretaceous magmatic arc on the southern margin of Eurasia was subducted to near-ultrahigh-pressure depths of 90-100 km beneath Eurasia during Eocene(?)-Miocene intracontinental convergence. Combined with Tibetan xenoliths and surface exposures, these xenoliths suggest that the Pamirs and Tibet share similar crustal architectures--a southern lower crust composed of Gondwanan crust overprinted by a Cretaceous magmatic arc and a northern lower crust composed of clastic sedimentary rocks--produced by parallel tectonic evolutions. The Pamirs xenoliths reached temperatures of ~1075°C after they underwent dehydration melting, enrichment metasomatism, and melt injection during subduction. This processing at extreme temperatures and pressures caused significant changes in physical properties of the subducted crust: melting decreased buoyancy whereas melt injection/metasomatism increased buoyancy