新疆库米什早泥盆世侵入岩时代、地球化学及大地构造意义

Geochronology, petrochemistry and tectonic implications of Early Devonian plutons in Kumux area, Xinjiang

详细的野外考察发现,新疆库米什地区发育强变形的花岗岩与辉长岩,花岗岩时代晚于辉长岩,两者相互交插,形成黑白相间的地貌格局。主要元素、微量元素和稀土元素分析表明,两者没有发生明显的地球化学同化混染。辉长岩轻稀土、不相容元素强烈富集,而重稀土、相容元素相对亏损,高的Sr含量及低的Rb/Sr比值表明岩浆来自强烈富集地幔源区,曾被源自沉积岩的流体混染,属于火山弧环境;HREE元素也存在分异,表明岩浆形成时涉及石榴石的形成。花岗岩锆石阴极发光结构研究显示,锆石内部普遍见浑圆状碎屑核,表明花岗岩可能为变质沉积岩经局部熔融形成;SHRIMPU_Pb测年结果表明岩体侵位时代为396±4Ma。结合其他地质资料,推测库米什变形侵入岩的形成与南天山洋的向北俯冲消减有关,早泥盆世时库米什地区属于俯冲带上盘活动陆缘环境,中、南天山地质单元的界线应位于库米什南、榆树沟蛇绿岩套以北。

Detailed field observations have identified basic and granitic plutons in Kumux area, whose intersections resulted in a white-black mosaic pattern at the surface. Geochemical analyses demonstrate that both granite and gabbro are strongly enriched in LREE and incompatible elements and depleted in HREE and other compatible elements. Very high Sr content and low Rb/Sr ratio of the gabbros can be attributed to some contamination of Sr-enriched fluids derived from sedimentary rocks. Such a geochemical feature suggests that the gabbros were derived from an enriched mantle wedge area in a magma arc environment. Weak fractionation of HREE indicates garnet crystallization during partial melting of the enriched mantle wedge. CL observations and SHRIMP U-Pb analyses of granitic zircons reveal that the granite is of S-type resulting from remelting of meta-sedimentary rocks at about 396 Ma. Taking into account the geological data from peripheral areas, the authors have reached the following conclusions： （1） the intensely deformed plutons of the Kumux area resulted from the northward sub- duction of the Paleo South Tianshan Ocean under the Central Tianshan terrane, （2） during the subduction, the Kumux area was located on the subduction-related continental magma arc zone, and （3） the boundary between the South Tianshan unit and the Central Tianshan unit should be re-located to the south of the Kumux and the north of the Yushugou ophiolite slice.