Geochemistry and zircon LA-ICPMS dating of Iqe River granitic gneiss,northern margin of Qaidam Basin
Geochemistry and zircon LA-ICPMS dating of Iqe River granitic gneiss,northern margin of Qaidam Basin
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发表时间：
2006
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通讯作者：
Cheng Dan-ling
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作者：
Cheng Dan-ling
地球化学研究表明，柴达木盆地北缘榴辉岩围岩伊克河花岗质片麻质SiO2、Al2O3、K2O、Na2O含量较高，TiO2、FeOt、MgO、MnO、CaO含量较低，Rittmann指数为1.92~2.36，氧化铝饱和指数ACNK为1.07~1.17，符合地壳源性s型花岗岩的特征。微量元素和稀土分析表明，该花岗质片麻岩富LILE、贫HFSE，稀土元素丰度较高，LREE分异明显，Eu负异常中至显著，与壳源、陆-陆同碰撞花岗岩相似。阴极发光（CL）研究表明，伊克河花岗质片麻岩中锆石呈核-缘（缘）织构，核为原岩残余物，缘有一定变质再结晶作用。LA-ICPMS原位微量元素研究表明，锆石经历了弱变质作用，后期也可能经历了热液蚀变。考虑到锆石的成分，选取变质作用或蚀变作用较弱的锆石测试结果，认为锆石年龄在（952±19）Ma左右，为花岗质片麻岩原岩受地壳重熔侵入的时期。另外，424 ~ 480 Ma可能是加里东期的变质期，此时花岗质片麻岩和榴辉岩的原岩都发生了变质作用，但变质作用较弱的片麻岩在早古生代造山运动中并没有与封闭的超高压榴辉岩一起经历深俯冲。在利用LA-ICPMS对复杂成因变质锆石进行U-Pb定年时，必须同时进行CL图像和原位成分分析，才能解释锆石的年龄。
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 crust-originated,continent-continent syn-collisional granite.The cathodoluminescence(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 recrystallization.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 remelting.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.