U-Pb age of detrital zircons from Early Permian "Shoushangou Formation" in Xi Ujimqin Banner, Inner Mogolia

Abstract

The "Shoushangou Formation", previously assigned to lower Permian, is widespread in southern Da Hinggan Mountains. The Tabinmiao profile of "Shoushangou Formation" in Xi Ujimqin Banner of Inner Mongolia is characterized by thick rhythmic in-terbeds comprising medium-to thin-layered mudstones and sandstones. Of them, the sandstones are mainly feldspathic lithic rocks. Zircons from sandstone in the upper sequence of the profile almost display typical structures of magmatic genesis. Eight (ca. 9% of the total 90 dated zircon grains) of the dated zircons yielded Proterozoic to early Paleozoic ages, ranging from 1490 Ma to 502 Ma. However, the yielded age data of the rest zircons are clustered in three groups: 383 Ma±3 Ma (n=3, ca. 3%), 365 Ma±4 Ma (n=6, ca. 7%) and between 337 Ma and 280 Ma (n=73). Four age peaks can be recognized between 337 Ma and 280 Ma, i.e., 289 Ma, 300 Ma, 309 Ma and 325 Ma. The obtained age data indicate the multi-provenance of the sandstone. Nevertheless, the youngest zircon age constrained the maximum depositional age of 289 Ma. According to the isotope age data of Dashizhai Formation, the authors infer that both Shoushangou Formation of this study and Dashizhai Formation belong to different facies of the same period. On the basis of U-Pb dating, it is deduced that the studied "Shoushangou Formation" was not the previously defined Shoushangou Formation, that is, the stratigraphic sequence and its correlation with the lower Permian strata in southern Da Hinggan Mountains need to be re-evaluated. Combined with the regional geology and timing of the magmatism, it is concluded that the detrital zircons of the dated "Shoushangou Formation" mainly came from the Paleozoic granitoids and volcanic rocks in the Xilin Hot-Xi Ujimqin Banner area, with the probable addition of a small amount of volcanic rocks of the Dashizhai Formation.