云南西盟曼亨花岗岩锆石U-Pb年龄与地质意义

Zircon U-Pb Ages and Its Geological Implications of Manheng Granite from Ximeng City, Yunnan Province, China

在1∶25万区域地质调查工作的基础上,对出露于西盟县新厂乡一带的曼亨花岗岩进行了岩石地球化学和精确的成岩年代学研究。岩石中w（Si O2）为72.73%~77.24%,w（K2O＋Na2O）=4.28%~9.01%（平均7.38%）,w（K2O）/w（Na2O）=0.65~20.84（平均2.67）,显示高硅和富碱的特点。标准矿物计算结果具较高q值,并出现较高c分子,标准矿物组合为c＋q＋or＋ab＋an＋hy,属于铝、硅过饱和系列。岩石的A/NCK为1.05~2.66（平均1.29）,属S型花岗岩。LA-ICPMS锆石U-Pb测年结果,显示曼亨花岗岩结晶年龄为446.1±2.5 Ma,表明该岩体形成于晚奥陶世。综合研究认为该期花岗岩具有同碰撞岩浆活动的特征,是泛非运动晚期冈瓦纳大陆汇聚过程其外围地区板块持续相互碰撞、造山作用的产物。

Based on 1： 250000 regional geological surveys, petrogeochemistry and diagenetic chronology of Menheng granite were studied in this paper. The results show that the contents of Si O2 and K2O＋Na2O range from 72.73 wt% to 77.24 wt% and 4.28 wt% to 9.01 wt%（average 7.38 wt%）, respectively. The ratios of K2O/Na2 O range from 0.65 to 20.84（mean 2.67）. These results suggest that Manheng granite has features of high Si O2 and alkaline-rich. C.I.P.W standard mineral results show high q and c values. The standard mineral assemblage is c＋q＋or＋ab＋an＋hy, belonging to the Al and Si supersaturation series. The A/NCK ratios are between 1.05 and 2.66（mean 1.29）, indicating that Manheng granite belongs to S-type granite. LA-ICP-MS zircon U-Pb dating of Manheng granite yields 446.1±2.5 Ma, suggesting that it had formed during the Late Ordovician. We consider that Manheng granite has characteristics of syn-collision magmatic active and is the product of periphery plate collision orogeny during the Late Pan-African and in the process of the convergence of Gondwana continent.