| 南秦岭小茅岭复式岩体地球化学特征及其构造意义 |
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| 引用本文：刘仁燕,李清山,牛宝贵,和政军,任纪舜.2018.南秦岭小茅岭复式岩体地球化学特征及其构造意义[J].地球学报,39(2):201-215. |
| DOI：10.3975/cagsb.2018.012501 |
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| 基金项目:中国地质调查局地质调查工作项目(编号: DD20160343-01);中央公益性科研院所基本科研业务费(编号: J1605) |
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| 中文摘要:南秦岭北缘小茅岭复式岩体主要由宋家屋场基性辉绿(辉长)岩, 迷魂阵、磨沟峡和叶家湾中性-偏酸性闪长岩组成, 形成于新元古代早期870~840 Ma。岩石地球化学研究显示, 宋家屋场基性岩属亚碱性系列岩石, 富集轻稀土元素及大离子亲石元素, Nb、Ta、Ti、P等具有弱的负异常, Zr、Hf等高场强元素的含量较高, Zr/Y、Ta/Yb比值较高, Zr/Nb比值较低, 与岛弧玄武岩的地球化学特征相似, 在构造环境判别图解中投于火山弧岩石区域, 一些元素的富集特征可能来源于受俯冲块体流体交代的岩石圈地幔。迷魂阵、磨沟峡及叶家湾闪长岩体均属准铝质亚碱性系列岩石, 具有岛弧火成岩的地球化学特征。Sr-Nd-Hf同位素地球化学分析显示, 宋家屋场岩体主要来自亏损地幔源区, 磨沟峡和叶家湾岩体主要为新生基性下地壳的部分熔融, 而迷魂阵岩体主要为古老地壳物质的部分熔融。上述特征表明小茅岭复式岩体各组成部分可能形成于870~840 Ma俯冲相关的岛弧构造背景, 俯冲块体脱水形成的流体交代上覆地幔楔, 这种地幔部分熔融形成了宋家屋场角闪辉绿(辉长)岩, 由于基性岩浆的加热, 诱发新生基性下地壳及原古老地壳物质部分熔融, 岩浆上升、侵位, 形成磨沟峡、叶家湾及迷魂阵岩体。结合陡岭地区~850–780 Ma变质作用及岩浆事件相关报导, 在南秦岭北缘小茅岭—陡岭隆起带上, 新元古代早期存在一期构造-岩浆-变质事件, 该事件的区域大地构造属性则有待进一步研究。 |
| 中文关键词:小茅岭复式岩体 岩石地球化学 Sr-Nd-Hf同位素 秦岭造山带 |
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| Geochemical Characteristics and Tectonic Implications of the Xiaomaoling Composite Intrusives in South Qinling Orogenic Belt |
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| Abstract:The Xiaomaoling composite intrusives, exposed in the northernmost part of South Qinling, are composed of Songjiawuchang diabase (grabbro) and Mihunzhen, Mogouxia and Yejiawan diorites with the age of 870~840 Ma. The geochemical analysis shows that the Songjiawuchang basic intrusion belongs to sub-alkaline series and is rich in LREE and LILE, slightly negative in Nb, Ta, Ti and P elements and has high content of Zr and Hf elements, high ratios of Zr/Y and Ta/Yb but low ratios of Zr/Nb. All the above characteristics are similar to those of typical Island Arc Basalts; in the tectonic discrimination diagrams, the Songjiawuchang samples fall in the VAB area. The enrichment of some elements might have resulted from lithospheric mantle metasomatized by the fluid of the subducted block. The Mihunzhen, Mogouxia and Yejiawan diorites belong to quasi aluminum sub-alkaline series and have the geochemical features of island arc magmatic rocks. Based on Sr-Nd-Hf isotopic analysis, the authors hold that Songjiawuchang intrusion was mainly derived from depleted mantle. The Mogouxia and Yejiawan diorites were derived from newly-created basic lower crust while the Mihunzhen diorite may be the product of ancient crust melting. All the geochemical data show that the Xiaomaoling composite intrusives were formed in an island-arc tectonic setting in 870~840 Ma. The metasomatized mantle wedge was partially melted and the produced magma was emplaced to form the Songjiawuchang intrusion. As a result of the basic magma heating, the overlying newly-created basic lower crust and ancient crust began melting and formed the Mogouxia, Yejiawan and Mihunzhen intrusions. Combined with the available ~850–780 Ma metamorphism and magmatism in the Douling area, the authors hold that there probably existed a period of early Neoproterozoic tectonic-magmatic-metamorphic events in the Xiaomaoling–Douling uplift of the South Qinling orogenic belt. The regional tectonic attributes of these events deserve further studies. |
| keywords:Xiaomaoling composite intrusives petro-geochemistry Sr-Nd-Hf isotope Qinling orogenic belt |
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