内蒙古乌拉特中旗准噶顺花岗闪长岩中锆石U-Pb年代学与岩石地球化学

Zircon U-Pb geochronology and geochemistry of Zhungashun granodiorites in Urad Zhongqi,Inner Mongolia

内蒙古准噶顺花岗闪长岩中锆石均呈自形-半自形,具有均匀的震荡生长环带和较高的Th/U比值(0.67~1.26),显示其岩浆成因特征。LA--ICP--MS锆石U--Pb定年结果表明,其206Pb/238U加权平均年龄为(433±2)Ma(早志留纪),该年龄代表了花岗闪长岩的结晶年龄。岩体地球化学分析结果SiO2为62.74%~64.50%,高CaO(4.47%~6.44%),高Al_2O_3(16.51%~17.22%),低K_2O(1.51%~2.02%),Na_2O(2.21%~3.54%),属于钙碱性系列,岩石富集大离子亲石元素(如K、Rb、Ba、Sr),亏损高场强元素(如Nb、Ta、Hf、Zr、Ti),稀土配分曲线右倾。低Mg~#值(Mg~#=39~43<45),低的相容元素含量(Cr、Ni含量分别为3.12×10^(-6)~24.6×10^(-6)、5.57×10^(-6)~11.6×10^(-6)),显示出壳源特点。综合区域同时代火成岩的研究成果,认为准噶顺花岗闪长岩形成于古亚洲洋俯冲消减的岛弧环境中。

Zircons in Zhungashun granodiorites are all euhedral to subhedral in shape with homogeneous oscillatory growth bands and high Th/U ratio（0.67 - 1.26）,which suggests their magmatic origin.LA--ICP--MS zircon U--Pb dating shows the weighted age of（433 ± 2） Ma with MSDW = 0.44,indicating the intrusion formed in Early Silurian.The rocks have a SiO2 content variety from 62.74% to 64.50%,high CaO（4.47% - 6.44%）and Al2O3（16.51% - 17.22%）,low K2O（1.51% - 2.02%） and Na2O（2.21% - 3.54%）,belonging to calc-alkaline series.The rocks are characterized by enrichment in LILE（such as K,Rb,Ba and Sr） and relatively depleting in HFSE（such as Nb,Ta,Hf,Zr and Ti）,and the chondrite-normalized REE patterns exhibit a right-dipping type.Low Mg#values（39 - 43 45） and compatible elements like Cr（3.12 × 10^-6- 24.6 ×10^-6） and Ni（5.57 × 10^-6- 11.6 × 10^-6） suggest that the primary magma was derived from lower crust.Combined with the data of igneous rocks of the same age,the authors conclude that the formation of Zhungashun granodiorites were related to the subduction of the Paleo-Asian oceanic plate.