LA-ICP-MS zircon U-Pb dating of the metamorphic rocks in the Ailaoshan tectonic belt, western Yunnan, and its geological implications

In order to determine the age of Ailaoshan Group and its tectonic affinity, the authors chose granodioritic gneiss (SM-15 and SM-22), its synchronic deformational granite vein intrusion (SM-18) in the high-grade metamorphic rock series and its proximate meta-sandstone (NO-1) in the low-grade metamorphic rock series as research objects and present new results of zircon U-Pb dating of four samples so as to do the accurate chronology tests. Two groups of ages were identified in sample SM-15 and SM-22. In sample SM-15, the older magmatic group yielded an age of 722~740Ma and the metamorphic younger group yielded an age of 29.9±0.5Ma. In sample SM-22, the older is 232.3±4.3Ma and the younger is 29.2±0.4Ma. In addition, the zircons from sample SM-18 yielded a concordant U-Pb ages of 27.19±0.36Ma. A total of 30 spots analyses of sample NO-1 defined four age groups at 246~276Ma, 600~800Ma, 800~1000Ma and >1000Ma respectively. From these data, the authors have drawn some conclusions: Firstly, the Ailaoshan Group consists of at least three stages of tectonic and magmatic activities, i.e., the 722-740Ma and 235±10Ma igneous rocks and sedimentary rocks, and the 27-32Ma igneous rocks. The metamorphic rocks in the study area are unlikely to be of the Yangtze Plate; Secondly, the peak metamorphism deformation time of Ailaoshan Group was about 27-32Ma.