摘要
京北南口至延庆古崖居出露了一套延伸40km、由钾玄质岩石和花岗质岩石组成的双峰式岩墙群。报道了岩墙群的同位素年龄,其K-Ar年龄为114～120Ma,Rb-Sr年龄为128Ma。对南口双峰式岩墙群的17个代表性样品进行了系统的元素地球化学分析,研究结果表明,酸性岩墙和基性岩墙是同时来自两个独立的岩浆房,岩浆来自EMⅠ型富集地幔。在此基础上,用岩浆不混熔的双扩散对流模式解释了两个层状岩浆房的形成,正是它们同时为双峰式岩墙群提供了物质来源。
A suite of bimodal dike swarms, stretched 40 km and consisting of shoshonite and granite rocks, are outcropped in the Nankou Guyaju area, north Beijing. Reported in this paper are the isotopic ages of the dike swarms, which are 114～120 Ma (by K Ar method) and 128 Ma (by Rb Sr method). Systematic element geochemical analysis is carried out of 17 representative samples from the Nankou bimodal dike swarms. The Rb/Ti ratios of acidic dikes and basic dikes differ from each other by one order, indicating that the two kinds of dikes come out of different magmatic chambers. But their partition patterns of trace elementsare basically consistent, both rich in LILE and LREE, deficient in HFSE, and with similar La/Ce ratios and similar isotopic Sr Nd values, reflecting a commonorigin of primordial magma. The low εSr(t)value (0.705 5～0.705 9)and negative εNd(t)value (-14.3～-10.5)of the two kinds indicatethat they are products of a comagmatic evolution coming from an EMⅠ type mantle. The basic dikes are richer in REE and with higher La/Yb ratio than the acidic ones. The fact is used in discussion on genesis of the rocks with a model of double diffusive convection for magma immiscibility to explain formation of the two layered magmatic chambers, which provide materials for the bimodal dike swarms. Nankou is located at the border of Yanshan Mt. and North China Basin, which are connected by the Nankou Early Cretaceous bimodal dike swarm in a common extensional settings. And the dike swarms make a bridge connecting the research on the deep magmatism and that on the shallow structural deformation. Such a study is valuable for further understanding the Yanshan movement in North China.
出处
《地球化学》
CAS
CSCD
北大核心
2001年第6期517-524,共8页
Geochimica
基金
国家自然科学基金(49872079
40072073)
中国科学院知识创新工程重大项目(KZCX1-07)
