Abstract: The Jiapila Formation in the Tuotuo River region, North Qiangtang depression is mainly composed of volcanic rocks intercalated with conglomerates. It is very different from the regional Jiapila Formation of primarily purplishred clastic rocks in either sedimentary characteristics or stratigraphic age. In this paper, we performed magmatic and detrital zircon UPb dating of the Jiapila Formation, involving volcanic rock and conglomerate in the Nangji and Kaixin Ridge, as well as sandstone of the Bagong Formation in the Quem Co area, to accurately determine the stratigraphic age of these rocks. Zircon UPb dating from the basaltic trachyandesite in the Jiapila Formation yielded a concordant age (weighted mean 206Pb/238U age) of 216.4±1.1 Ma (n=28, MSWD=0.39). The youngest UPb age of detrital zircons from polymictic conglomerate and sandstone samples gave weight mean ages of 222.2±3.7 Ma (n=2) for the Jiapila Formation and 222.8±1.9 Ma (n=9) for the Bagong Formation, respectively. Therefore, we placed the eruption age and the maximum depositional ages for these rocks at the Late Triassic Norian. Combined with regional historical data and lithological column of the latest well with the continuous sequences, we redefined volcanic rock of the Jiapila Formation as Nadigangri Formation, and confirmed that the conglomerate was formed by the tectonic uplift in the Late Triassic. During the mid to late stage of the Late Triassic, most areas of the North Qiangtang was subjected to the uplift and erosion. In the hiatus, a layer of weathering crust on limestones was formed at the top of Xiaochaka Formation in the west, along with the formation of an exposure surface on top of the Jiezha Group in the east, as well as formation of the distinct components of conglomerates, indicating varying degree of weathering between west and east. Due to expansion of the BangongNujiang Tethys Ocean, the North Qiangtang rifted with massive volcanic eruptions, causing widescale deposit of the volcanicsedimentary sequences of the Nadigangri Formation in the east and west. Furthermore, exposure of the Nadigangri Formation matched well with the rifting basement of the Upper Triassic strata, which indicates the volcanicsedimentary event was a localized event related to rifting in the region.

Key words: zircon U-Pb age, Jiapila Formation, synrifting, volcanicsedimentary, North Qiangtang, Late Triassic