U–Pb geochronology of paragneisses and metabasite in the Xitieshan area, north Qaidam Mountains, western China: Constraints on the exhumation of HP/UHP metamorphic rocks

Abstract

The Xitieshan HP/UHP metamorphic unit is a polyphase paragneiss–orthogneiss–metabasite complex preserving relics of an early eclogite facies metamorphism (as deduced from the presence of clinopyroxene + plagioclase symplectite after omphacite in metabasite) followed by decompression through the granulite and amphibolite facies. Paragneisses contain garnet, kyanite (partially replaced by sillimanite), feldspar, quartz and biotite, with minor rutile and zircon. The metabasite consists of garnet, amphibole and plagioclase with relict clinopyroxene + plagioclase symplectite. Two main high-temperature metamorphic stages are distinguished: (1) a high-pressure granulite stage, characterized by the assemblage garnet–kyanite–K-feldspar–plagioclase–quartz–rutile in the paragneisses and garnet–clinopyroxene–plagioclase–quartz in the metabasite; (2) a medium–low pressure granulite to upper amphibolite facies stage, leading to the formation of sillimanite in the paragneisses and overgrowth of amphibole on Cpx–Pl symplectite in the metabasite. SHRIMP U–Pb analyses of metamorphic zircons, in conjunction with LA-ICPMS REE data and rare mineral inclusions in zircons, show that the high-pressure granulite metamorphism recorded by these rocks occurred between 451 and 461 Ma and the medium–low pressure granulite–upper amphibolite facies metamorphic overprint between 423 and 427 Ma, suggesting about 30 Ma residence time in medium–lower crustal levels after an earlier eclogite facies metamorphism (ca. 480 Ma). Based on comparison with other HP/UHP metamorphic units in the North Qaidam Mountains (NQD), these results suggest diachronous subduction and exhumation in the different units of the NQD.