摘要
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。
Tiaojishan Formation in Western Hills of Beijing was dated at Late Jurassic based on the Ar-Ar ages for tachyandesites (all plateau ages and isochrone ages of plagioclase are respectively 148.91±2.98 Ma and 146.60±2.93 Ma). Geochemistry of elements and isotopes show that volcanic rocks of Tiaojishan Formation are mainly of high-K calc-alkaline series characterized by indistinct Eu anomaly, relative enrichment of large-ion lithophile elements such as.Rb, Sr and Ba, relative depletion of high field-strength elements (e.g. Nb, Ta, Ti, Zr, Hf ), and low Nd, Sr and Pb ratios, suggesting the features of island -arc and active continental margin and enriched mantle. The authors think that volcanic rocks of Tiaojishan Formation were derived from partial melting of old-age (Archean) basaltic rocks of the lower crust and the crust-mantle transitive zone by the underplating of basaltic magma during intrcontinental orogeny.
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2001年第2期123-133,共11页
Acta Petrologica et Mineralogica
基金
国土资源部"九五"重大基础项目! (95 0 1111)
国家自然科学基金资助项目! (4 96 0 2 0 37)
国家杰出青年科学基金项目! (4 972 5
关键词
髫髻山组
下地壳-壳幔过渡带
玄武质岩石
晚侏罗世
火山岩
陆内造山带
北京
岩浆
地球化学特征
Tiaojishan Formation
partial melting
the lower crust-the crust-mantle transitive zone
basaltic rocks
intrcontinental orogen zone
Western Hills of Beijing
