Redefinition and Formation Age of the Tanjianshan Group in Xitieshan Region,Qinghai

The Tanjianshan Group, which was previously divided into a, b, c and d formations, has been controversial for a long time. It mainly distributes in the northern margin of Qaidam Basin and is an important early Paleozoic greenschist facies metamorphic volcanic sedimentary rock formation. Detailed field investigation and zircon LA‐ICPMS U‐Pb dating of the key strata suggest that the original lower part of a Formation (a‐1) versus the original middle upper of d Formation (d‐3 and d‐4), the original upper part of a Formation (a‐2) and b Formation versus the original lower part of d Formation (d‐1 and d‐2) of Tanjianshan Group are contemporaneous heterotopic facies volcanicclasolite deposit, respectively. The former formations formed during the middle‐late Ordovician (463–458 Ma), while the latter ones formed in the late Ordovician (about 445 Ma). The original c formation of Tanjianshan Group, which formed after 430 Ma, is similar to the Maoniushan Formation of Kunlun Mountains and north Qaidam Basin. According to the rules of stratigraphic division and naming, new stratum formations of Tanjianshan Group are re‐built and divided into Duancenggou (O_1–2td), Zhongjiangou (O_2–3tz) and Xitieshan (O₃tx) formations. The original c Formation is separated from Tanjianshan Group and is renamed as the Wuminggou Formation (S₃‐D₁w), which shows a discordant contact with underlying Tanjianshan Group and overlying Amunike Formation (D₃a). The zircon U‐Pb age frequency spectrogram of Tanjianshan Group indicates three prominent peaks of 430 Ma, 460 Ma and 908 Ma, which is consistent with the metamorphic and magmatic crystallization ages obtained from para‐ and orthogneisses in north Qaidam HP‐UHP metamorphic belt, implying that strong Caledonian and Jinningian tectonic and magmatic events have ever happened in North Qaidam.