New discovery and tectonic significance of Cambrian and Ordovician strata in the Zuogong Block, eastern Xizang

The lack of a definitive discovery of Early Paleozoic sedimentary cover in the Zuogong Block is one of the reasons for controversy over the division of stratigraphic system and the structural framework in eastern Xizang. This paper reports the results of detrital zircon U-Pb dating and regional stratigraphic correlation of two newly identified shallow metamorphic rock series, originally classified as Triassic strata, in the Cuoga and the Guoxuepu areas of the Zuogong Block, and further discusses the structural attributes of the Zuogong Block. Mica quartz schist, mica schist, marble, and meta quartz sandstone assemblages have been identified in the original Upper Triassic Guoxuepu Formation (Adula Formation) in the Cuoga area. The mica quartz schist exhibits a youngest clastic zircon age of 548 Ma, indicating a Cambrian sedimentary age according to regional stratigraphic lithology and age correlation. The original Upper Triassic Guoxuepu Formation (Jiapila Formation) in the Guoxuepu area consists of slate and metaquartz sandstone assemblages, with its sedimentary age revised to the Early Ordovician, based on the youngest clastic zircon age of 491 Ma in the meta quartz sandstone. The difference in the youngest age components of the detrital zircons from the two newly identified strata reflects changes in the sedimentary source areas between the Cambrian and Ordovician. Based on regional data, it is believed that the Zuogong Block is comparable to the South Qiangtang and Baoshan Blocks in terms of Early Paleozoic strata, Early Paleozoic magmatic events, and Late Paleozoic glacial-marine diamictite, in addition to Pan-African detrital zircons. During the Prototethys evolution stage, the Zuogong Block was part of the northern margin of the Gondwana continent and served as a link between the South Qiangtang and Baoshan Blocks. This result indicates that the extension of the Longmuco-Shuanghu suture zone in eastern Xizang lies between the Zuogong Block and the Changdu Block.