The collision between the Yili and Tarim blocks of the Southwestern Altaids: Geochemical and age constraints of a leucogranite dike crosscutting the HP–LT metamorphic belt in the Chinese Tianshan Orogen

Abstract

A ca. 600 m-long, 0.5–20 m-wide NW–SE trending granite dike crosscuts the high pressure–low temperature (HP–LT) Tianshan metamorphic belt, the foliation of which is parallel to the main ENE regional trend in the Chinese South Tianshan Orogen. It is mainly composed of plagioclase, K-feldspar, quartz, muscovite, biotite and secondary chlorite, while fluorite, zircon and xenotime occur as accessories. The immediate country rock is a quartz–biotite–plagioclase schist, which grades several tens of meters away from the granite dike into a chlorite–mica–albite schist. The latter schist is intimately intercalated with blueschist layers and boudins. The A/CNK value of the granite dike samples varies from 1.15 to 1.27 indicating a strongly peraluminous composition. CaO/Na 2O ranges from 0.06 to 0.17 and Al 2O 3/TiO 2 from 240 to 525, similar to the ratios of strongly peraluminous (SP) granites exposed in ‘high-pressure’ collision zones such as the Himalayas. A zircon U–Pb age of 285 Ma was obtained for the granite dike, thus constraining the upper limit for the age of HP–LT metamorphism. The petrological and geochemical data suggest that the SP leucogranite dike intruded during the exhumation of overthickened crust in the post-collisional setting between the Yili (–Central Tianshan) and Tarim blocks. The dataset presented here in conjunction with previously published data corroborate that the HP–LT metamorphism must have occurred earlier than the Permian in the Tianshan Orogen. Therefore, the collision between the Yili (–Central Tianshan) and Tarim blocks and the final amalgamation of the Southwestern Altaids must have been terminated in Late Paleozoic and not in Triassic times as previously suggested.