Geological characteristics and geochronology of Jiaokong metamorphic rocks from the Nanshan area, Qinghai Province, and their geological implications

The Jiaokong metamorphic rocks in northern Dabuzuhu Mountain of Nanshan area, Qinghai Province, were originally assigned to Early-Middle Triassic Longwuhe Formation. Through detailed field work, the authors examined the output state and rock assemblage of the Jiaokong metamorphic rocks. Then the provenance and the formation age of this metamorphic suite were studied, with the purpose of investigating the evolution of the study area during Late Paleozoic to Early Mesozoic period. This metamorphic suite was composed of arkosic sandstone, quartzite, marble, and mica-quartz schist. The suite has a total thickness of over 3186.3m, and can be divided into 3 sections from bottom upward. The detrital zircon ages can be divided into 5 age clusters, i.e., 250~304Ma, 405~546Ma, 649~1077Ma, 1402~1620Ma and 1861~1990Ma. The 250~304Ma age cluster can be further divided into two sub-clusters of 250~269Ma and 289~304Ma. Taking into account of previous study results and regional tectonic-magmatic activities, the authors confined the depositional time limit of the Jiaokong metamorphic rocks to Permian, and its sedimentary sources might have mainly come from Caledonian and Jinningian magmatic arc in Qilian orogenic belt, with Hercynian magmatic arc in the structural belt of North Qaidam basin also providing some sedimentary materials. Nanshan area of Qinghai experienced a complete process from intracontinental rifting, basin extension and oceanic subduction to continental collision in the Late Paleozoic-Early Mesozoic period.