Triassic eclogites from central Qiangtang, northern Tibet, China: Petrology, geochronology and metamorphic P–T path

Abstract

High-pressure (HP)/low-temperature (LT) metamorphic rocks, such as eclogite and blueschist, are generally regarded as an indicator of subduction-zone metamorphism. Eclogites have recently been discovered in the central Qiangtang Block. Their occurrence is highly significant to the understanding of the closure of the Paleo-Tethys and tectonic evolution of northern Tibet. We report the results of petrological, mineralogical and geochronological investigations of these rocks, and discuss their tectonic implications. The Qiangtang eclogite occurs as blocks and lenses in Grt–Phn schist and marble, and is composed of garnet, omphacite, phengite and rutile. Eclogitic garnet contains numerous inclusions, such as glaucophane and phengite in the core, and omphacite in the mantle or inner rim. In strongly retrograded eclogite, the omphacite is replaced by glaucophane, barroisite and albite. Four stages of metamorphic evolution can be determined: (1) prograde blueschist facies; (2) peak eclogite facies; (3) decompression blueschist facies and (4) retrograde greenschist facies. Using the Grt–Omp–Phn geothermobarometer, a peak eclogite facies metamorphic condition of 410–460°C and 2.0–2.5GPa was determined. Zircon U–Pb dating gave ages of 230±3Ma and 237±4Ma for two eclogite samples. The ages are interpreted as the time of eclogite facies metamorphism. Moreover, 40Ar/39Ar dating of phengite from the eclogite and Grt–Phn schist yielded ages about 220Ma, which are probably indicative of the time of exhumation to the middle crust. We conclude that the HP/LT metamorphic rocks were formed by northward subduction of the Paleo-Tethys Ocean and they marked a Triassic suture zone between the Gondwana-derived block and Laurasia.