新疆阿吾拉勒山西段群吉萨依铜矿辉绿玢岩脉继承锆石年龄及地质意义

新疆阿吾拉勒山西段是伊犁古裂谷的重要组成部分, 出露较多辉绿岩脉, 其中, 群吉萨依铜矿辉绿玢岩脉受隐爆角砾岩筒控制。笔者对该岩脉开展了锆石U-Pb年龄测试和岩石地球化学分析, 分析结果表明:辉绿玢岩中锆石以继承锆石为主, 其年龄值主要集中在1 781~1 823Ma, 17个测点的207Pb/206 Pb加权平均值为(1 798.9±6.1)Ma, 这一年龄值与西天山地区前寒武纪结晶基底形成时代一致。岩石整体高Fe、Ti, 富Na, 贫K, 不相容元素含量较低, 具弱Eu异常, 富集高场强元素, 亏损大离子亲石元素, 暗示岩浆来源类似于弱亏损地幔, 但受到了少量地壳物质的混染。结合前人研究成果, 该辉绿玢岩脉形成于板内裂谷伸展构造环境, 可能与上地幔玄武质岩浆底侵作用有关, 地幔岩浆上升过程中捕获了大量基底岩石中的岩浆锆石和变质锆石, 从侧面证实了伊犁微板块早-中元古代结晶基底的存在。

Significance and Inherited Zircon U-Pb Age of Diabase-porphyrite in Qunjisayi Copper Deposit,Western Awulale Mountain,Xinjiang

As an important part of the ancient Yili rift, the western Awulale Mountain possesses many diabase dykes including the Qunjisayi diabase-porphyrite which is controlled by volcanic crypto explosive breccia pipes. The diabase-porphyrite dyke contains many inherited metamorphogenic zircons which yield weighted average 207Pb/206Pb ages of (1798.9±6.1) Ma. This age is consistent with the age of Precambrian Basement in Western Tianshan Mountain. The diabase-porphyrite dyke is characterized with higher FeO, TiO2 and Na2O content, little K content, lower incompatible elements content and weak negative δEu anomaly. The rocks are enriched with HFSE such as Th, U, Ta, Nb, Zr and Hf, depleted of LILE such as Rb, Ba, K, Sr and Cs. The petrogenesis of Qunjisayi diabase-porphyrite dyke indicates that the magma is derived from weak depleted mantle mingled with a little crust rocks. All the research results indicate that the Qunjisayi diabase-porphyrite dyke is generated from continental rift setting which is relevant with the underplating of the upper-mantle basaltic magma. With the basaltic magma ascending, it captures a great deal of metamorphogenic zircons from Precambrian basement rocks, which indirectly testifies the existence of early-medium Proterozoic crystalline basement in Yili microplate.