Origin of Late Silurian A-Type Granite in Wulonggou Area, East Kunlun Orogen:Zircon U-Pb Age,Geochemistry, Nd and Hf Isotopic Constraints

Wang Yilong; Li Yanjun; Wei Junhao; Li Huan; Han Yu; Zhou Hongzhi; Huang Xiaokun; Ke Kunjia

Abstract：Late Silurian A-type granite is reported for the first time in the Wulonggou gold district,eastern segment of the Kun-lun orogenic belt.This paper presents(1)laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)U-Pb age for the Wulonggou A-type granite to determine precisely the time of the magmatism,(2)geochemical,Nd and Hf isotope data for the Wulonggou A-type granite to constrain the petrogenesis and tectonic setting.The LA-ICP-MS U-Pb analyses of zircon from the Wulonggou A-type granite yielded a weighted mean 206Pb/238U age of 420±3 Ma,indicating that it was emplaced in the Late Silurian.All rock samples are enriched in SiO2(76.0%—78.4%),K2O(4.64%—5.22%)and Na2O(2.93%—3.25%),but with relatively low concentrations of FeOT(0.98% — 1.45%),MgO(0.11% — 0.22%)and CaO (0.27%—0.79%).Samples are also enriched in large ion lithophile elements(Rb,K,La)and light rare earth elements,but depleted in high field strength elements(Nb,P,Ti)and heavy rare earth elements with strong negative Eu anomalies(Eu/Eu?=0.09—0.12).The 104×Ga/Al ratios of the Wulonggou A-type granites vary from 3.09 to 3.15.Mineralogy and geo-chemistry of the rocks show an affinity with aluminous A-type granite.TheεNd(t)values of whole-rock range from —2.5 to—2.2 with corresponding two-stage Nd model ages ranging from 1 339 to 1 365 Ma.TheεHf(t)values of zircon vary from —2.8 to +2.1 and two-stage Hf model ages range from 1 269 to 1 583 Ma.The Nd and Hf isotopic data imply that different source materials have contributed to the magma genesis.Integrated geological,geochemical and isotopic data suggest that Wulonggou A-type granite is most likely generated via the mixture of asthenosphere-derived magma and the ancient crustal materials.The A-type granite was marked by A2type and was formed in post-collisional extensional tectonic environment.In combination with the A-type granites such as those in the Helegangnaren and Binggou,it is concluded that the eastern Kunlun area had begun its post-orogenic extension stage since Late Silurian,and this stage continued at least about 34 Ma.