桂东北牛庙闪长岩和同安石英二长岩:岩石学、锆石SHRIMP U-Pb年代学和地球化学
Niumiao and Tong'''' an intrusive bodies of NE Guangxi: Petrology,zircon SHRIMP U-Pb geochronology and geochemistry
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摘要: 桂东北牛庙和同安岩体分别由闪长岩和石英二长岩组成。锆石的SHRIMP U-Pb年龄分别为163±4Ma和160± 4Ma。岩石以富铝、富碱、高钾、富含Rb、Ba、Th、U、Pb、Sr等大离子半径亲石元素(LILE)及富含REE、Nb、Ta、Zr、Hf等高场强元素(HFSE)为主要特征,属富钾系列或钾玄岩系列。其不相容元素的分布特征为亲OIB型,主要来源于富集的岩石圈地幔的熔融。两岩体的常量元素、微量元素和Sr-Nd同位素组成特征,反映了它们的原始岩浆经历过相当充分的分离结晶和相当程度的地壳混染,即通过AFC方式而形成。岩体中广泛分布的微细粒状暗色包体是共存的更偏基性的岩浆与寄主岩浆不完全混合的残留,其成分的多样性和相互过渡关系,反映了不同包体母岩浆在形成和演化途径等方面的差异性。岩体形成于燕山早期华南后造山阶段大陆地壳拉张减薄的构造环境,软流圈地幔沿超岩石圈深断裂的上涌和底侵,是造成富集的岩石圈地幔和中下地壳熔融并形成本区闪长质和花岗质岩浆的主要机制。
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关键词:
- 高钾岩系 /
- 锆石SHRIMP定年 /
- 富集的岩石圈地幔来源 /
- 暗色包体 /
- 后造山拉张环境
Abstract: The Niumiao and Tong' an intrusive bodies are composed of diorite and quartz monzonite, respectively. Their zircon SHRIMP U-Pb ages are 163 4Ma and 160 4Ma, respectively. These rocks are enriched in Al, alkalies, K, Rb, Ba, Th, U, Pb, Sr etc. LILE's and REE, Nb, Ta, Zr, Hf etc. HFSE's, and with an OIB affinity in incompatible elements distribution pattern. They belong to the high-K or the shoshonitic association and were essentially generated by partial melting of the enriched subcontinental lithospheric mantle and via an AFC model. The widespread mafic microgranular enclaves are remains from mingling of more mafic magmas with host dioritic or quartz monzonitic magmas. The variation and transition in chemical compositions of enclaves reflect differences in formation and evolution paths of their parental magmas. These intrusive bodies were emplaced at the post-orogenic stage under an extensional and thinning environment within the South China continental crust. Upwelling and underplating of the asthenospheric mantle along the trans-lithospheric deep faults were responsible for melting of the enriched subcontinental lithospheric mantle and overlying middle-lower crust, and for generating the early Yanshanian dioritic and granitic magmas in this region. -
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