西昆仑大同西岩体成因:矿物学、地球化学和锆石U-Pb年代学制约  

Origin of Datongxi pluton in the West Kunlun orogen:Constraints from mineralogy,elemental geochemistry and zircon U-Pb age

作　　者：高晓峰[1] 校培喜[1] 康磊[1] 奚仁刚[1] 过磊[1] 谢从瑞[1] 杨再朝[1] 

机构地区：[1]国土资源部岩浆作用成矿与找矿重点实验室,中国地质调查局西安地质调查中心,西安710054

出　　处：《岩石学报》2013年第9期3065-3079,共15页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41002020);陕西省自然科学基金项目(2011JQ5008);中国地质调查局国土资源大调查研究项目(1212011085034)联合资助

摘　　要：大同西岩体出露于西昆仑造山带西段,主要岩性为高钾钙碱性的石英闪长岩和石英二长岩-二长花岗岩。石英闪长岩中普遍发育椭球形或卵形暗色镁铁质包体,包体岩性主要为闪长岩,石英闪长岩的SiO2的含量为56.6%~65.3%,Mg#为0.45~0.46,包体SiO2的含量为52.9%左右,Mg#为0.48。相对于石英闪长岩,石英二长岩-二长花岗岩具有较高SiO2的含量（SiO2=65.0%~73.3%）和较低Mg#（Mg#=0.36~0.44）。微量元素分析数据表明,石英闪长岩与暗色包体的REE球粒陨石标准化图呈右倾型,Eu负异常不明显,富集大离子亲石元素,亏损高场强元素。石英二长岩-二长花岗岩具有明显的Eu负异常,强烈亏损Nb-Ta、Ti等元素。温压计算显示石英闪长岩为相对低温（638~768℃）的中-深成岩体（平均深度为8.8km）,石英二长岩-二长花岗岩具有相对高的结晶温度（752~771℃）。锆石U-Pb年代学研究表明,石英二长岩形成年龄为470±1.2Ma,略晚于前人报道的石英闪长岩侵位年龄。石英二长岩-二长花岗岩与石英闪长岩来源于不同的岩浆,二者之间不存在分异关系。结合区域构造演化历史,早期的石英闪长岩为大洋板片俯冲过程中地幔楔部分熔融岩浆的分异产物,晚期的石英二长岩-花岗岩则来源于碰撞后伸展阶段由于软流圈地幔物质上涌加热导致变泥质岩石熔融的结果。大同西复式岩体的成因指示了西昆仑造山带在早奥陶纪经历了由俯冲向碰撞后伸展的构造体制转换。The Datongxi pluton outcropping in western part of the West Kunlun orogen, consisted of high-K calc-alkaline quartz diorite, quartz monzonite and granite. The quartz diorites contain abundant mafic microgranular enclaves （MMEs） of different shapes. The quartz diorites span a SiO2 range of 56. 5% - 65.3% and Mg range of 0.45 N 0. 46, characterized by moderate LREE/HREE fractionation, strong LILE enrichment and depleted Nb-Ta and weakly Eu depletion. Relatively to the quartz diorites, the quartz monzonites and granites have high content of SiO2 （65.0% -73.3% ）and low Mg~ （0. 36 N 0. 44）, stronger Nb-Ta, Ti and Eu depletion. The calculated P-T conditions are 638 -768~C and 1.7 ~ 4. 3kbar for the quartz diorites and 752 -771~C for the quartz monzonites and granites, respectively. Zircon U-Pb dating results give an emplacement age of 470 ＋_ 1.2Ma for the quartz monzonites, slightly younger than the emplacement ages of 478 ~473Ma for the quartz diorites. The younger quartz monzonite-granite suite cannot be formed through differentiation of the older quartz diorite, indicating that both of them were derived from different melting sources. The quartz diorites and MMEs were formed through melting of the metasomatized mantle wedge in a subduction setting, while the younger quartz monzonite-granites were derived from metasedimentary protoliths in response to post-collisional extension and thermal supply from the underplating of the upwelling mantle-derived magmas. The origin of the Datongxi felsic intrusive complex may record the tectonic evolution of western Knnlun orogenic belt from the oceanic slab subduction to the post-collisiona1 extension during Early Ordovician time.

关 键 词：矿物学 地球化学 岩石成因 大同西岩体 奥陶纪 西昆仑造山带 

分 类 号：P588.122[天文地球—岩石学] P597.3[天文地球—地质学]