Age, genesis and geological significance of the metarhyolite in the southern margin of the southern Tianshan Orogenic belt: constraints from zircon U-Pb age and trace elements

　　
The composition and age of metamorphic rocks serve as crucial indicators for comprehending the nature, formation, and evolution of the basement of orogenic belts. In this study, comprehensive petrographic examination, LA-ICP-MS zircon U-Pb dating, and trace element analysis were conducted on metarhyolite interlayers within Archean schist located in the northern region of Korla, situated in the eastern sector of the South Tianshan orogenic belt. The geochronological results show that the protolith was mainly formed in 412-418 Ma at the beginning of the early Devonian. Combined with the regional structural analysis, it is believed that the metamorphism occurred at the end of the late Paleozoic. Based on the trace element characteristics of zircon, it is estimated that the crustal thickness during rhyolite eruption was less than 35 km, with evidence of plagioclase separation and crystallization in the magmatic source area. Additionally, the zircon Ti thermometer suggests a high magmatic crystallization temperature (＞ 800℃), indicative of a high-temperature magmatic origin that possibly formed within a plate tectonic environment. In conjunction with earlier investigations, it is deduced that the southern Tianshan orogenic belt could have constituted a segment of Tarim during the Early Paleozoic era, with its northern margin representing an active continental margin. Influenced by subduction and retreat along the northern margin, the southern Tianshan region underwent significant back-arc extension, likely occurring in the Late Silurian and early Devonian periods. The identified rhyolite in this study is postulated to be a component of the sedimentary fill within the back-arc rift basin during that time.