西秦岭造山带礼县地区中川岩体LA-ICP-MS锆石U-Pb定年及其构造环境

中川复式岩体位于西秦岭中带,与成矿关系密切,主体岩性为黑云母二长花岗岩,一般认为其形成于印支期。对中川岩体采集样品并进行了LA-ICP-MS锆石U-Pb定年和岩石化学分析。结果表明,岩体钾钠含量较高,Na2O/K2O〈1;里特曼指数δ值为1.70～2.17,主体属高钾钙碱性系列;轻稀土元素富集,具中等程度的负Eu异常;K、Th、Rb等大离子亲石元素富集,同时显示出低Sr、低Yb的地球化学特征,形成于地壳加厚的构造环境。锆石U-Pb定年指示了08-49号样品黑云母二长花岗岩的形成时代最早为264.4Ma±1.3Ma,即二叠纪末期,略早于前人研究的结果。结合其构造环境,认为西秦岭从二叠纪末就开始了陆陆俯冲和地壳加厚的造山活动,并一直持续到三叠纪。

LA-ICP-MS zircon U-Pb dating and tectonic setting of Zhongchuan intrusion,Lixian area,western Qinling orogen

Located in the middle of western Qinling orogen,the Zhongchuan intrusion is closely related to mineralization.The main intrusive body consists of biotite adamellite,which is generally considered to have been formed in Indo-Chinese epoch.LA-ICP-MS zircon U-Pb ages and major and trace elements of whole-rock were determined for Zhongchuan intrusion.The results show that the intrusion belongs to high-K calc-alkaline series with high content of Na2O,K2O and low content of MgO.Chondrite-normalized REE patterns are characterized by strong LREE enrichment and variable degrees of negative Eu anomalies（δEu=0.60～0.67）;The ORG normalized trace element patterns show that the intrusion is enriched in LILE（K,Th,Rb） and depleted in HFSE（Ta,Nb,Y and Yb）.The Zhongchuan intrusion basically shows the characteristics of low content of Sr and Yb and was formed in the syn-collision circumstances.U-Pb age determination of zircon indicates that the earliest biotite adamellite intruded at 264.4±1.3Ma in the Late Permian,earlier than 219-232Ma.Taking into account the tectonic environment,the authors hold that the continent-continent collision of western Qinling area began at the end of Permian and lasted to Triassic.