青海省祁连县西出露一个似斑状正长花岗岩体,运用LA-ICP-MS方法,对似斑状正长花岗岩中的锆石进行U-Pb同位素分析.测年结果显示,似斑状正长花岗岩的形成年龄为166.6±2.4Ma,表明其形成于燕山期中侏罗世,继承锆石显示前寒武纪源区的时代信息.岩石地球化学特征表明,似斑状正长花岗岩具有高硅、富碱特点,属于过铝质高钾钙碱性岩,具弱负Eu异常,富集Rb、Th、K,亏损Ba、Nb、P、Ti,Rb/Sr值平均为1.86,反映出壳源特点,为S型花岗岩.%The porphyaceous syenogranite pluton is exposed in the west of Qilian Country,Qinghai Province.The authors conduct-ed LA-ICP-MS zircon U-Pb dating of the porphyaceous syenogranite and the result shows that the age is 166.6±2.4Ma,suggesting the Middle Jurassic of Yanshanian period.Several inheritable magmatic zircons yielded the age information of the Precambrian peri-od. Geochemical data show that the porphyaceous syenogranite is silica-enriched in composition with high content of alkali, and hence it is a peraluminous granite and belongs to the high-K calc-alkaline series.It is characterized by enrichment of Rb,Th,K,de-pletion of Ba,Nb,P,Ti,and weak negative anomaly of Eu,with the average Rb/Sr ratio being 1.86,which reflects the characteris-tics of the crust source and indicates S type granite.
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