Constraints of detrital zircon U-Pb ages and Hf isotopes on the provenance of the Triassic Yidun Group and tectonic evolution of the Yidun Terrane, Eastern Tibet

Eastern Tibet to the west of the Yangtze Block consists of the Yidun and Songpan–Ganzi Terranes, separated by the Ganzi–Litang suture zone. The Yidun Terrane includes the Zhongza Massif to the west, but the eastern part of the Yidun terrane is covered by the Yidun Group extending from the south (Shangri-La region) to the north (Changtai region). The Yidun Group, from the base upward, includes the Lieyi, Qugasi, Tumugou and Lanashan formations, which are mainly composed of volcanic-flysch successions. Based on the ages of volcanic interlayers and plutonic intrusions, depositional ages of the Qugasi and Tumugou formations are considered to be slightly older than 230Ma and ca. 220–230Ma respectively, which are prominently older than the previous estimates. The Yidun Group in the Changtai region has two prominent detrital zircon age peaks at 400–480 and 880–980Ma and a minor peak at 2.45–2.50Ga. This pattern suggests a detritus source from the Zhongza Massif, which was a micro-continent separated from the western Yangtze Block. In contrast, the Yidun Group in the Shangri-La region has various zircon age spectra among different formations. The Qugasi Formation in this region has detrital zircon age patterns similar to the Yidun Group in the Changtai region. However, the overlying Tumugou Formation shows distinct age peaks at Triassic (220–240Ma), Neoproterozoic (~720–880Ma), and Paleoproterozoic (~1.75–1.90Ga). This age pattern is similar to that of the Xikang Group of the Songpan–Ganzi Terrane to the east. The detrital zircon age difference between the Qugasi and Tumugou formations in this region indicates a transition of sedimentary sources from the Zhongza Massif to locally distributed Triassic magmatic rocks at ~230Ma. It is thus suggested that the Songpan–Ganzi Terrane may have been connected to or collided with the southern part of the Yidun Terrane during the Late Triassic, whereas the Songpan–Ganzi Terrane and the northern part of the Yidun Terrane were still separated by the Ganzi–Litang Ocean.