新疆北部乌恰沟地区镁铁质侵入岩的年代学、地球化学和Sr-Nd-Pb同位素组成:对地幔源区特征和深部过程的约束

Geochronology,geochemistry and Sr-Nd-Pb isotopic composition of mafic intrusive rocks in Wuqiagou area,north Xinjiang:Constraints for mantle sources and deep processes.

阿尔泰造山带东段的乌恰沟地区发育大量镁铁质侵入岩（岩脉或小岩体），包括闪长玢岩和具堆积结构的角闪辉长岩和橄榄辉长岩。锆石SHRIMP U-Pb同住素定年结果说明这些镁铁质岩浆的侵位时间为257．4±5．3Ma，与峨眉山玄武岩相近（259±3Ma）。在微量元素原始地幔标准化图上，它们共同的特征是富集大离子亲石元素，亏损Nb和Ta，具K和Pb的正异常，LREE相对HREE富集，即具孤火山岩的地球化学特征。同时它们之间还存在互补的地球化学特征，如闪长玢岩存在Ba、Sr、Eu、P和Ti的负异常，橄榄辉长岩具有Ba、Sr、Eu、P的正异常，而角闪辉长岩具有Ti的正异常。如果扣除围岩混染和后期流体交代的影响，这些岩石的Sr-Nd-Pb同位素组成基本一致，因此闪长玢岩和两种辉长岩分别代表同源岩浆因结晶分离作用形成的演化岩浆和堆晶岩。乌恰沟晚二叠世镁铁质侵入岩和新疆北部早二叠世与铜镍矿有关的基性-超基性杂岩体都具有与孤火山岩相似的地球化学特征，反映了被俯冲改造的陆下岩石圈地幔的地球化学特征。但是与后者相比，前者的Mg^#值、εNd（t）值和La／Nb比明显偏低，暗示存在软流圈地幔来源物质的显著贡献。综合阿尔泰乌恰沟地区镁铁质侵入岩的年代学、地球化学和同位素组成特征，我们推测该区在晚二叠世存在地幔柱与岩石圈地幔之间的相互作用。

Mafic intrusive rocks in Wuqiagou area, eastern part of Altai Orogenic belt, include diorite porphyrite, olivine gabbro cumulates, and hornblende gabbro cumulates. Zircon SHRIMP dating results an age of 257.4 ± 5.3Ma for the diorite porphyrite, which is near-synchronous with the late-Permian plume represented by Emeishan basalts. The geochemistry of these rocks is similar to that of arc igneous rocks, including enrichment in LILEs, depletion in Nb and Ta, positive K and Pb anomalies, enrichment in LREE relative to HREE in spider diagram. Meanwhile, the signatures for some elements of these rocks are complementary each other, e.g. the diorite porphyrite has negative Ba, Sr, Eu, P and Ti anomalies in spider diagram, the olivine gabbro has positive Ba, Sr, Eu and P anomalies, and the hornblende gabbro has positive Ti anomaly. After the affect of assimilation and fluid metasomatism from wall rocks are corrected, these rocks show consistent Sr-Nd-Pb isotopic compositions. Therefore, these mafic rocks might have a common source in mantle and resulted as the evolved magma and cumulate in crust, respectively. The arc-like geochemistry of these rocks suggest a subduction-modified sub-continental lithospheric mantle source, but the lower values of Mg^#,εNd （ t）, as well as the lower La/Nb ratios, than that of maiic plutons of the early Permian in North Xinjiang, suggest that not only sub-continental lithospheric mantle but also asthenosphere contribute to their petrogenesis. Therefore, geochronology, geochemistry, and isotopic signatures of the mafic rocks in Wuqiagou support that plume-lithosphere interactions exist in North Xinjiang at the late Permian.