Detrital Zircon U-Pb Geochronology of Molasse Sedimentary Rock Series in Xianghuanggou Section, Northeastern Margin of North Qilian Mountains and Its Geological Significance

The ocean-continent pattern, sedimentary tectonic evolution and the relationship between adjacent blocks in the Qin-Qi-He crosslinking area on the southwest margin of the North China Craton since the Early Paleozoic have been the focus of research in recent years. In this paper, LA-ICP-MS zircon U-Pb geochronological analysis is used to analyze a set of coarse clastic molarite sedimentary rocks, which were previously thought to belong to Silurian and Devonian but lack of fossil records, from Xianghuanggou section in Nanhuashan area, northeastern margin of the North Qilian tectonic belt. Clastic rock samples were collected and detrital zircon U-Pb dating analysis was performed. The results show that the U-Pb concordant age of the youngest zircon is 230.2±2.6 Ma, and the concordant age of several younger zircons is 232±5 Ma, 232±3 Ma and 236.7±2.4 Ma, respectively. U-Pb weighted age of the youngest zircon is 233.2±5.1 Ma, and the earliest depositional age of the host strata is the Late Triassic. Combined with the age spectrum of detrital zircons from the adjacent stratigraphic units, it is concluded that the main body of the molarite sedimentary strata in the study area belongs to the Upper Triassic sedimentary stratigraphic units. It does not support the traditional understanding of the Silurian Devonian system in the past. According to the comparative analysis of regional age spectrum, the Late Triassic coarse clastic molarite sedimentary rocks in Xianghuanggou section of Nanhuashan Mountain are mainly derived from the North Qinling and Alxa areas in the Early Mesozoic, and the Qilian-North Qinling areas in the Early Paleozoic. The source of Precambrian detrital material is a bidirectional mixture source between the North China plate and the Qilian-North Qinling orogenic belt. The conglomerate deposits were mainly controlled by the tectonism of the Qin-Qi collisional orogenic belt in the Indosinian period, and should belong to the sedimentary tectonic response of the final collisional closure process of the Qin-Qi trough in the Indosinian period.