Zircon U-Pb geochronology of Zenong Group volcanic rocks in Coqen area of the Gangdese, Tibet and tectonic significance

Extensive Zenong Group volcanic rocks are zonedly E-W distributed in the middle-northern Gangdese, Tibet. Well-exposed Zenong Group volcanic rocks in Coqen, western Gangdese are selected for zircon U-Pb dating by using the SHRIMP II system at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing and the LA-ICP-MS system at the China University of Geosciences, Wuhan to construct the geochronological framework of the Zenong Group volcanic activity for regional comparison. Most zircons separated from the Zenong Group volcanic rocks exhibit striped absorption and obvious oscillatory zoning, and a few ones show core-rim structure. One dacite sample and one rhyolite sample from the Zenong Group volcanic rocks overlying the Middle Permian Xiala Formation limestone in northwestern Daxiong, Coqen yield LA-ICP-MS zircon U-Pb age of 130 +/- 1Ma and SHRIMP zircon U-Pb age of 129 +/- 3Ma, respectively. One dacite sample from the Zenong Group volcanic rocks underlying the elastic rocks of lower Cretaceous Duoni Formation by fault contact in northeastern Daxiong, Coqen yields younger age cluster of 121 +/- 1Ma ( include growth rim) and older age cluster of 131 +/- 1Ma (include core). One rhyolite sample from southwestern Coqen yields LA-ICP-MS U-Pb age of 111 +/- 1Ma, coeval with the emplacement age of granitoids in Coqen area. High-quality zircon U-Pb ages reported in this study indicate that the Zenong Group volcanism began at ca. 130Ma, and ceased at ca. 110Ma with a volcanic duration of ca. 20Ma. Regional comparison indicates that the Zenong Group volcanism extending for ca. 1000km E-W is synchronously commenced at ca. 130Ma. Present geochronological age dates combined with the distribution of contemporaneous magmatism in a regional context suggest that the Early Cretaceous magmatism in the middle-northern Gangdese cannot be generated by the low-angle or flat-slab subduction of Neo-Tethyan oceanic crust.