摘要
北大巴地区广泛分布了呈北西-南东向产出的基性岩墙群,主要侵位于下古生界。对该套岩系的地球化学分析表明,w(K2O+Na2O)、w(TiO2)高,总稀土元素质量分数高并呈轻稀土元素富集型,大离子亲石元素明显富集,Ta、Nb的质量分数远高于洋中脊玄武岩,具大陆裂谷玄武岩的地球化学特征。采用LA-ICP-MS法对北大巴基性岩墙中的岩浆锆石进行了U-Pb同位素定年,获得岩体结晶年龄为(431.0±3.2)Ma。地球化学及年代学数据表明,北大巴基性岩墙群具有大陆裂谷玄武岩的地球化学特征,表明北大巴早古生代伸展构造受到扬子板块北缘的幔源裂谷作用制约。在裂谷演化过程中,上地幔物质上涌造成岩石圈板块拉伸减薄,使北大巴地区中上地壳发生强烈构造伸展并形成大面积呈线状分布的基性岩墙群,并且该处(431.0±3.2)Ma的年龄值代表了在早志留世北大巴裂谷盆地的伸展裂陷幅度达到最大。
Basic dike swarms are widely distributed NW-SE, intruding the Early Paleozoic strata in North Daba Mountains. The geochemistry analysis show that this rock series share similar characteristics of the average mass fraction of continental rift basalt. It is sound that K2O＋Na2O is concentrated in the major elements with high TiO2 and rare-earth content. Also the general high rare-earth content is associated with high concentration of light rare-earth and the incompatible large-ion lithophile elements, while the content of Ta and Nb is much higher than MORB. The basic dike swarms crystalline dated （431.0±3.2） Ma by the LA-ICP-MS zircon U-Pb dating. These geochemistry characters and dating above indicate that the basic dike swarms in the study area show the characters of the continental rift basalt. This helps to infer that the extensional tectonics developed constrainedly in the Early Paleozoic by the mantle-source rifting that happened at the boundary of paleocontinent on the northern margin of Yangtze Plate. In the process of rifting, material in the upper mantle uplifts resulted in the extension and thinning of lithospheric plate, generating intense extensional tectonics developed in the middle-upper crust of Daba area and then producing the large group of linear basic dike swarms. The age of （431±3.2） Ma represents the extension of this rift to its peak in Early Silurian.
出处
《地质科技情报》
CAS
CSCD
北大核心
2009年第3期19-26,共8页
Geological Science and Technology Information
基金
中国石油化工股份有限公司勘探南方分公司项目"南大巴构造特征及构造演化研究"(P02009)
