Zircon U-Pb age for rhyolite of the Maoniushan Formation and its tectonic significance in the East Kunlun Mountains
东昆仑牦牛山组流纹岩锆石U-Pb年龄及构造意义

The Maoniushan Formation of molasse, formed in a post-orogenic volcano-sedimentary basin at the Shuinichang area in the East Kunlun Mountains, rest unconformably on pre-Devonian strata and its forming time is important for determining the closure of the Early Paleozoic Ocean in this region. We made zircon U-Pb dating for rhyolite interlayers from different levels of this formation by LA-MC-ICPMS. The rhyolite (sample B743-2) above the basal conglomerate of the Maoniushan Formation on the northern margin of the basin yielded an average zircon U-Pb age of 423.2±1.8Ma and the rhyolite (sample B820-1) above the basal conglomerate of the formation on the west margin of the basin yielded an average zircon U-Pb age of 408.2±2.4Ma. The rhyolite interlayers (samples B705-1 and B656-1) from detrital rocks of the middle-upper formation yielded two average zircon U-Pb ages: 404.9±4.8Ma and 399.6±2.8Ma. These ages should represent the forming times for different levels of the Maoniushan Formation and thus provide a constrain to the forming time of the formation in this area, that is, 400~423Ma. Therefore, it is suggested that the closure time of the Early Paleozoic Ocean should be in the Late Silurian. The discovery of 2468~920Ma zircon of the Proterozoic age in the rhyolite shows that the metamorphic basement south of the East Kunlun Mountains would be similar to that of the Yangtze plate and would develop during the formation of the Rodinia supercontinent 900~1000Ma ago.