Provenance and tectonic setting of the Kunyang Group in central Yunnan Province

Formation age, sedimentary environment and source rock characteristics of the Kunyang Group are problems of considerable controversy. In order to identify the provenance and tectonic setting of the Kunyang Group in central Yunnan, the authors analyzed the sedimentary assemblage and sedimentary facies of the Kunyang Group, and conducted LA-ICP-MS detrital zircon U-Pb dating of 3 metamorphic sandstone samples from the Kunyang Group, which yielded different youngest concordant ages for Huangcaoling Formation, Heishantou Formation and Meidang Formation, being 984.0 Ma, 945.0 Ma and 954.0 Ma respectively. Age spectrum of these detrital zircons reveals a wide age range from Archean to Neoproterozoic with prominent peak at ~1.0 Ga, ~1.35 Ga and ~1.73 Ga and the second peak at ~2.44 Ga, mainly concentrated on ~1.35 Ga and ~1.73 Ga. It is inferred that the Kunyang Group source area mainly experienced 1, 1.35 and 1.73-2.44 Ga tectonic thermal events, and the age of the Dahongshan Group in southwest Yangtze Block is about 1.7Ga. The period of tectonic thermal event in Greenvillian is from 1.0 Ga to 1.3Ga. The authors also analyzed the geochemical characteristics of 20 extremely low-grade metamorphic rocks from Kunyang Group. The result shows that source rocks of Kunyang Group are quartzose cyclic sediments, felsic rocks and some mafic rocks formed in active continental margin or continental island arc. It is concluded that, from late Mesoproterozoic to early Neoproterozoic (0.95-1.0 Ga), the Kunyang Group was formed in the foreland basin with its provenance mainly from the Dahongshan Group in southwestern Yangtze block and Grenville-aged island arc.