西天山晚石炭世岩浆混合花岗岩的确定及其地质意义——来自玉奇布拉克和乌图精河岩体的LA-ICP-MS锆石U-Pb年龄和岩石地球化学证据

摘要

The Yuqibulake and Wutujinghe plutons in the Boluokenu area are typical mixing granitoids with numerous mafic enclaves in West Tianshan Orogen.Poikilitic,disequilibrium magmatic textures and needle-like apatite are very developed in the mafic enclaves,suggesting an origin of two-end member magma mixing.In contrasting to the host granitoids,the enclaves show relatively lower SiO2,K2O and higher CaO,FeOT,MgO.While,both of the host granitoids and enclaves have a similar characteristics of trace elements with enrichment in LILE,such as Rb,Th,U and K,depletion in Sr,Nb,Ta,P and Ti,presenting a result from the two-end magma mixing.The Yuqibulake granites and mafic enclaves yielded zircon U-Pb ages of 301 Ma and 308 Ma,respectively.The ages of 303 Ma and 298 Ma were obtained from Wutujinghe granites and enclaves,suggesting that the coeval mantle-derived basic magmatism was occurred with this period of granitoid magmatism.The host granitoids have positive eHf(t) varied from +4.20 to +8.33 for Yuqibulake pluton and from +4.97 to +7.09 for Wutujinghe pluton with TDMC of 782 ～ 1045 Ma and 863 ～998 Ma,respectively,indicating they were partially melted from the early Neoproterozoic crust materials.The enclaves from both plutons also have positive εHf(t) of +2.75 to +6.41 and +4.63 to +7.92,respectively,similar to those εHf(t) and εNd(t) from coeval basic dykes in western Tianshan Mountais.Combined with the coeval granitoids with higher εHf(t),εNd(t) and medium n(87Sr)/n(86Sr),we proposed that they were formed by mixing of the magmas from partial melting of early Neoproterozoic crustal materials and from slightly depleted lithospheric mantle.This magmatism evolved from High-K calc-alkaline granitoids in Late Carboniferous to K-rich granite and A-type granite in Early Permian,representative of products formed in a transitional tectonic setting from post-collision to extension.The crustal extension resulted in mantle upwelling to produce mantle-derived basic magma and then trigged partial melting of crustal materials,leading to a strong and wide granitoid magmatism by the mixing of two-end member magmas,and being a significant vertical crust growth in west Tianshan Mountais.%西天山博罗科努地区的玉奇布拉克和乌图精河岩体中发育大量暗色闪长质包体,包体发育矿物相互包裹的嵌晶及岩浆冷凝矿物不平衡结构,同时出现淬冷形成的针状磷灰石,指示岩体形成过程经历了二元岩浆的混合作用.包体较寄主花岗岩有明显低的SiO2、K2O,高CaO、FeOT和MgO,但二者的微量元素高度一致,相对富集Rb、Th、U、K等LILE,贫HFSE,亏损Sr、Nb、Ta、P和Ti等元素,为二元岩浆混合作用所致.经LA-ICP-MS锆石U-Pb定年,两个岩体寄主花岗岩和包体分别获得301Ma、308 Ma和303 Ma、298 Ma的形成年龄,指示同期不同源岩浆活动并存.寄主花岗岩的εHf(t)值分别为+4.20～+8.33和+4.97～+7.O;Hf陆壳模式年龄7 CDM为782～1045 Ma和863～998Ma,揭示它们主要源自新元古代早期陆壳物质.包体的εHf(t)值分别为+2.75～ +6.41和+4.63～+7.92,与区域上同期基性岩脉正的εHf(t)和εNd(t)值相当.结合区域上同期花岗岩类高εHf(t)和εNd(t)、中等n(87Sr)/n(86Sr)推断,该期花岗岩是新元古代早期陆壳物质部分熔融的岩浆与被消减带组分改造的略亏损岩石圈地幔岩浆混合作用的结果,并构成由晚石炭世高钾钙碱性花岗岩向早二叠世富钾花岗岩/或A型花岗岩转换的岩浆演化系列,表明晚石炭世—早二叠世期间西天山造山带转入后碰撞晚期向伸展转化的构造环境.陆壳伸展导致幔源基性岩浆上侵,诱发陆壳部分熔融产生壳源岩浆,二者混合形成晚古生代以来西天山最为广泛的花岗岩浆活动,成为区内一次重要的地壳垂向增生事件.