秦岭南召岩体中花岗岩脉的锆石U-Pb定年:对燕山期构造环境的约束  

The U-Pb zircon dating of the granite dike in Nanzhao pluton and it's constraints on tectonic setting in Yanshanian

作　　者：张昕[1] 吴才来[1] 陈红杰[1] ZHANG Xin;WU Cailai;CHEN Hongjie(Institute of Geology, Chinese Academy of Geology Sciences, Beijing 100037, China)

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《中国地质》2017年第5期938-958,共21页Geology in China

基　　金：国家深部探测技术与实验研究专项(Sino Probe-05-05);国家自然科学基金项目(41472063);中国地质调查局项目(1212011120160;12120114079901)联合资助~~

摘　　要：通常认为,花岗岩体中的花岗岩脉与岩体是同一次岩浆活动的产物,是深部演化的岩浆沿着岩体的裂缝侵入后冷凝形成的。南召岩体位于北秦岭东部,近年来在岩体中发现一条宽约5 m的花岗岩脉。岩脉呈紫红色,斑状结构,斑晶为石英。LA-MC-ICP-MS锆石U-Pb定年得出该岩脉的形成时间为(119.6±0.7)Ma,与岩体的年龄(452.3±6.2)Ma相差很大。锆石Hf同位素测试结果表明,岩脉的源岩可能主要是新生地壳,结合地球化学研究分析得出该岩脉呈现I型花岗岩的特性。该岩脉的年龄、Hf同位素和地球化学特征同邻近的伏牛山岩体相近,推测岩脉的源岩可能和伏牛山岩体的源岩相同。根据研究结果及区域地质构造分析,认为燕山期由于太平洋板块的俯冲作用和陆內拉张作用,导致扬子板块及华北板块沿秦岭造山带形成一系列断裂。岩浆沿着断裂上侵,大部分形成了伏牛山岩体,小部分沿着周围的裂隙侵入形成岩脉,并在侵位的过程中与寄主岩发生物质交换。It is usually believed that plutons and dikes resulted from the same magmatic activity.Deeply evolved magma invaded into the pluton from cracks.Nanzhao pluton is located in the east of North Qinling Mountain,in which a5m wide granite dike was discovered recently.The dike has purple,porphyritic texture and the phenocrysts are quartz.The U-Pb zircon LA-MC-ICP-MS dating shows the age of the dike is(119.1±0.8)Ma,which is quite different from the age of the pluton(452.3±6.2)Ma.The Hf isotope analysis shows that the source magma maybe a mixture of juvenile crust.This result and the geochemical study indicate that the granitic dike exhibits the characteristics of I-A type granite.The similar age data,Hf isotope analysis and geochemical characteristics suggest that the dike in Nazhao pluton may have the same source magma as adjacent Funiu Mountain.According to the previous results and the analysis of regional geological background,the authors hold that the subduction of the Pacific plate occurred in the Yanshanian period,whereas the Yangtze plate and the North China plate experienced pull-apart along Qinling orogenic belt.A tectonic environment of the pull-apart was formed and,as a result,a series of faults were formed between the plate.After that,the granite magma invaded onto the surface along the faults.Most of the magma formed Funiu Mountain,while a little magma formed a dike in Nanzhao pluton,with the exchange of elements during the invasion.Therefore,the granite dike in Nanzhao pluton has tectonic implications.

关 键 词：花岗岩脉 锆石U-Pb定年 Hf同位素分析 岩石成因 南召 

分 类 号：P588.121[天文地球—岩石学]