柴北缘鱼卡河花岗质片麻岩的地球化学特征和锆石LA-ICPMS定年  

Geochemistry and zircon LA-ICPMS dating of Iqe River granitic gneiss,northern margin of Qaidam Basin

作　　者：林慈銮[1] 孙勇[1] 陈丹玲[1] 第五春荣[1] 

机构地区：[1]西北大学大陆动力学国家重点实验室,西北大学地质学系,陕西西安710069

出　　处：《地球化学》2006年第5期489-505,共17页Geochimica

基　　金：国家自然科学基金(40472043);国家重点基础研究发展计划项目(G1999075508)

摘　　要：岩石地球化学研究表明,柴北缘鱼卡河超高压榴辉岩围岩花岗质片麻岩的SiO2、Al2O3、K2O和Na2O含量较高,而TiO2、FeOt、MgO、MnO和CaO含量较低,里特曼指数为1.93～2.36,铝饱和指数ACNK值为1.07～1.17,显示出明显的壳源S型花岗岩的特点。在微量元素方面,岩石富集大离子亲石元素,亏损高场强元素,具有轻稀土元素富集和中等的Eu负异常,也显示了壳源S型花岗岩的特征。阴极发光(CL)图像显示该花岗质片麻岩的锆石具有核-幔(边)结构,核部为原岩残留、幔部显示弱的变质重结晶。LA-ICPMS锆石原位微量元素研究显示,本区锆石经历了弱的变质重结晶作用,且可能遭受到后期热液蚀变作用的改造;参考锆石的微量元素组成特征,挑选受变质重结晶和热液改造较弱的锆石的同位素进行测试,结果获得鱼卡河花岗质片麻岩的原岩形成时代为(952±19)Ma左右的新元古代,是地壳重熔作用的产物;同时获得该片麻岩的可能变质年龄为424～480Ma,表明该片麻岩的原岩可能与榴辉岩的原岩共同经历了早古生代加里东期的变质作用,但由于片麻岩的变质程度较低,应是在榴辉岩的折返过程中构造叠置在一起的。利用LA-ICPMS对具有多阶段演化历史的变质锆石定年时,必须同时进行锆石CL图像和原位微量元素分析,才可能对所获年龄给出合理的解释。The geochemical investigation shows that Iqe River granitic gneiss, country rock of eclogite, from the northern margin of Qaidam Basin contains higher SiO2, Al2O3, K2O and Na2O, and lower TiO2, FeOt, MgO, MnO and CaO, with Rittmann index of 1.92±2. 36 and alumina saturation index ACNK of 1.07 ± 1.17, which are compatible with the characteristics of crustal derived S-type granites. Trace element and REE analyses show that the granitic gneiss is rich in LILE and poor in HFSE, and has relatively high REE abundances, visible fractionation of LREE and medium to pronounced negative Eu anomalies, which are also similar to those of crnst-originated, continent-continent syn-eollisional granite. The eathodolumineseenee （CL） investigations reveal that the zircons from Iqe River granitic gneiss show core-rim （margin） texture, and the core is the residue of protolith while the rim underwent some metamorphic reerystallization. LA-ICPMS in situ trace elements studies indicate that the zircons experienced weak metamorphism and may also undergo hydrothermal alteration later. Considering the composition of the zircons, we choose the test results of zircons with weaker metamorphism or alteration, and acquire an age around （952±19） Ma which is considered as the time when the protolith of granitic gneiss intruded by crust remehing. In addition, the age between 424 Ma and 480 Ma is probably the metamorphism time in Caledonian when the protolith of granitic gneiss and eclogite are both metamorphosed, but the gneiss with weaker metamorphism has not experienced deep subduction together with the enclosed ultra-high pressure eclogite during early Paleozoic orogeny. When having U-Pb dating for the metamorphic zircons with complex genesis by LA-ICPMS, both CL images and in situ composition analysis are needed before explaining the acquired age.

关 键 词：花岗质片麻岩 地球化学 锆石定年 柴达木盆地 

分 类 号：P595[天文地球—地球化学] P597[天文地球—地质学]