Discussion on Newly Disintegrated Metavolcanic Rock Series from the Dakendaban Group in Xiaosaishiteng Mountain Area, Qinghai Province

The Dakendaban Group(Pt₁D.) is the crystalline basement of Oulongbulak block in the northern margin of Qaidam Basin. It is a set of deep metamorphic rock series with metamorphic degree of amphibolite facies and formed in Paleoproterozoic. Based on the 1:50000 special geological survey, a new mylonitization and intermediate-acid metavolcanite series has been delineated from the original Dakendaban group in the Xiaosaishiteng Mountain, south of Sugan Lake, Oulongbulak block in the northern margin of Qaidam Basin, which is obviously different from the rock assemblages of Dakendaban group. LA-ICP-MS zircon U-Pb dating shows that the meta-rhyolite protolith is (435.2±1.5) Ma and meta-dacite protolith is (430.6±2.6) Ma, indicating that the mylonitic metamorphic intermediate-acid volcanic series formed in the Early Silurian. Comprehensive analysis disintegrated it from the Dakendaban Group and temporarily classified it as the Tanjianshan Group. The formation age of mylonitized granitic dikes intruding into meta-dacite is (269.3±1.8) Ma, and the age of upper intercept age is (1 607±14) Ma, which represents the formation age of the parent rock or the age of the ancient surrounding rock captured in the process of magmatic intrusion. In addition, the tectonic thermal event age of mylonitized meta-dacite is (265.7±4.0) Ma, which is later than the formation age of left-lateral shearing and crinkling mylonitized granitic dikes, indicating that the newly disintegrated metavolcanic rock series occurred in the late Variscan after the intrusion of granitic dikes.