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    湘东北新元古代花岗岩体锆石SHRIMP U-Pb年龄及地球化学特征

    Zircon SHRIMP dating and geochemical characteristics of Neoproterozoic granites in southeastern Hunan

    • 摘要: 提要:在湘东北岳阳—平江一带分布有许多新元古代花岗岩体,对其中的张邦源岩体进行锆石SHRIMP U-Pb年龄测定,获得(816±4.6) Ma的年龄。花岗岩体岩石基性程度较高,SiO2平均为70.25%;低钾高钙,CaO平均为2.38%;全碱较低,Na2O+ K2O平均为5.97 %,且Na2O>K2O;ASI值平均1.16(0.97~1.29)。总体属镁质过铝质钙碱性花岗岩系列。不相容元素Rb、Th含量较高,贫Ba和Nb。稀土元素总量较低,平均130 μg/g;轻稀土相对富集,(La/Yb)N比值在6.76~8.50;铕亏损不明显,δEu值为 0.54~0.78。εNd(t)值较高且变化范围较窄,为-1.5~1.6, T2DM1.35~1.60 Ga;δ18OV-SMOW值为8.1‰~11.1‰。岩石具典型的I型花岗岩或ACG型花岗岩特征,为壳-幔混合型成因,岩浆主要起源于亏损地幔源,并伴有地壳物质加入。地球化学特征显示为岛弧火山岩系,为会聚板块边缘(俯冲带)岩浆作用的产物,不具备地幔柱岩浆特性,不能作为Rodinina超大陆裂解的标志。

       

      Abstract: Abstract:There occur Neoproterozoic granitic bodies in Yueyang-Pinjiang area, northeast Hunan. Zircon SHRIMP U-Pb dating of the granite from Zhangbangyuan pluton was performed, yielding an age of 816±4.6 Ma. The rocks are lightly basic with an average SiO2 content of 70.25%, have low potassium and high calcium with an average CaO content of 2.38 %, and contain poor alkali with an average Na2O+K2O content of 5.97 % and the feature Na2O>K2O. The ASI values are 0.97~1.29 with an average of 1.16. Thus the granites are of the magnesian peraluminous calc-alkaline series. Rb and Th are rich, while Ba and Nb are poor. The rocks have low ΣREE values with an average of 130 μg/g, (La/Yb)N values of 6.76~8.50, δEu values of 0.54~0.78, εNd (t) values of -1.5~1.6, T2DM ages of 1.35~1.60 Ga, and δ18OV-SMOW‰ values of 8.1~11.1. According to the above data, the granites should belong to I-type or ACG formed by crust-mantle magmatic mixing, and the main source was derived from the depleted mantle with the participation of a small amount of crustal matter. Geochemical characteristics suggest that the granites are of the island arc volcanic system, and derived their materials from magmatism in the convergent margin of the plate instead of from mantle plume magmatism. Hence they cannot serve as the indicator of Rodinia Breakup.

       

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