Zircon U-Pb dating and geochemical characteristics of detrital rocks and their constraints on the provenance of Middle Jurassic Wanbao Formation in Tuquan basin

The detrital rocks of Middle Jurassic Wanbao Formation mainly include conglomerates, sandstones and mudstones. Among the sandstones, lithic sandstones are dominant, and feldspathic lithic sandstones and debris feldspar sandstones take the second place. This kind of distribution implies characteristics of near source sedimentation. The rock assemblage comprising gravels, debris and heavy minerals implies that the parent rocks of the clastic rocks consisted of rhyolite, andesite, granite, argillo-arenaceous rocks and some intermediate to low-grade metamorphic rocks. Geochemical characteristics of the clastic rocks indicate that the rocks in the provenance region were mainly felsic and formed in the environment of continental island arc and active continental margin. 100 U-Pb data were obtained by LA-ICP-MS method for the zircons in detral rocks from Wanbao Formation in this study, and the majority of the grains from the sandstone samples in Wanbao Formation are clustered in the ages of 171Ma±7Ma～1221Ma±23Ma. Among them, the younger age (171Ma±7Ma) represents the oldest sedimentary age, two of the older age (1221Ma±23Ma，504Ma±10Ma) may display a source of crystalline palaeobasement, and the other ages (between 214Ma±5Ma and 359Ma±9Ma) belong to the periods between Triassic and Carboniferous. According to these geological characteristics, the authors hold that the east and the south paleouplifts near Tuquan basin had already existed when sedimentary strata in Wanbao Formation were formed, and the provenances of Wanbao Formation were mainly from Dashizhai Formation, Zhesi Formation and Paleozoic intrusive rocks.