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对北京西山髫髻山组粗安岩斜长石Ar-Ar年龄测定获得全坪年龄和等时线年龄分别为148.91Ma± 2 .98Ma和 146 .6 0Ma± 2 .93Ma(相关系数r =0 .9997) ,本文将其时代划为晚侏罗世。元素 -同位素地球化学显示 ,西山髫髻山组火山岩主要为高钾钙碱性系列 ,低Ti、K ,高Al、Ca和Na ,轻稀土元素富集 ,Eu异常不明显 ,大离子亲石元素Rb、Sr、Ba等相对富集 ,高场强元素Nb、Ta、Ti、Zr、Hf相对亏损 ,Nd、Sr和Pb同位素比值较低 ,反映出火山岩形成于岛弧或活动大陆边缘 ,其源岩为富集地幔。据此认为髫髻山组火山岩形成于陆内挤压环境 ,由中生代玄武质岩浆底侵于下地壳 -壳幔过渡带诱发古老 (太古宙 )的玄武质岩石部分熔融形成的。
The Arlington age and isochron age of the coarse-grained amphibolite from the Qianshan Mountain Group in Xishan, Beijing, were 148.91Ma ± 2.98Ma and 146.60Ma ± 2.93Ma, respectively (correlation coefficient r = 0. 9997), this article delimits the epoch of Late Jurassic. Elemental-isotopic geochemistry shows that the volcanic rocks in the Xishan Qiaomishan Formation are mainly high-K calc-alkaline series with low contents of Ti, K, Al, Ca and Na, light rare earth elements (REEs), anomalous Eu anomaly, large ion lithophile Rb Sr, Ba and so on. Relatively high depletion of high-field elements Nb, Ta, Ti, Zr and Hf and relatively low Nd, Sr and Pb isotopic ratios reflect the formation of volcanic rocks on the island arc or active continental margin. To enrich the mantle. Therefore, the volcanic rocks of Qiaomishan Formation formed in the inland compression environment and were formed by the partial melting of the ancient (Archean) basaltic rocks induced by the basaltic magmatic emplacement of the Mesozoic in the lower crust-mantle transitional zone.