西天山昭苏北部大哈拉军山组火山岩中辉长岩体的形成时代、地球化学特征及地质意义  

Gabbroic pluton in the Dahalajunshan Formation volcanic rocks from northern Zhaosu,western Tianshan: Age,geochemistry and geological implications

作　　者：林靓[1,2] 钱青[1,3] 王艳玲 高俊[1] 江拓[1,2] 刘新[1,2] 

机构地区：[1]中国科学院地质与地球物理研究所、矿产资源重点实验室,北京100029 [2]中国科学院大学,北京100049 [3]中国科学院青藏高原地球科学卓越创新中心,北京100101 [4]美国罗切斯特大学地球与环境科学系,罗切斯特NY14627

出　　处：《岩石学报》2015年第6期1749-1760,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40872057;41472058);基金委重大项目(41390445);杰出青年基金项目(41025008)联合资助

摘　　要：西天山昭苏北部侵入于大哈拉军山组火山岩层上部的辉长岩体具有富集大离子亲石元素（Rb、Sr、Ba）、亏损高场强元素（Nb、Ta）、轻重稀土分馏等地球化学特征,与火山岩围岩具有相似的不相容元素和Sr-Nd同位素特征,其母岩浆可能由俯冲流体交代的富集岩石圈地幔部分熔融形成,晚石炭世之前南天山洋盆向伊犁-中天山板块之下的俯冲可能导致了岩石圈的富集作用。利用不相容元素进行地球化学模拟计算,结果表明辉长岩成分由50%~80%的堆晶矿物（单斜辉石、斜长石）与50%~20%的玄武质熔浆组成。辉长岩体的Cameca锆石U-Pb年龄为311.3±2.3Ma,与伊犁-中天山板块晚石炭世伊什基里克组火山岩的时代大致相当,略晚于西天山榴辉岩的峰期变质时间。辉长岩的时代进一步限定该地区大哈拉军山组火山活动应在早石炭世晚期结束,下石炭统阿克沙克组沉积岩应形成于320~311Ma之间。与辉长岩同时期的岩浆岩在伊犁-中天山板块广泛分布,形成于俯冲结束之后挤压环境向拉张环境过度的构造环境。The gabbroic rocks intruded in the upper part of the Dahalajunshan volcanic formation from northern Zhaosu,western Tianshan,are characterized by enrichment in large-ion-lithophile elements（ LILE） such as Rb,Sr and Ba,strongly depletion in highfield-strength elements（ HFSE） such as Nb and Ta,and fractionation of rare earth elements（ REE）. The gabbroic rocks are analogues to the surrounding volcanic rocks in terms of incompatible elements and Sr-Nd isotopes. Their parental magmas were probably produced by partial melting of enriched lithospheric mantle that had been metasomatized by subduction fluids due to the northward subduction of the South Tianshan oceanic crust beneath the Yili-Central Tianshan block before the early stage of the Late Carboniferous. Geochemical modeling using incompatible race elements indicates that the gabbroic rocks are composed of 50% ~ 80% cumulated crystals of clinopyroxene and plagioclase and 50% ~ 20% basaltic magma. Cameca zircon U-Pb dating yielded a weighted mean206 Pb /238 U age of311. 3 ± 2. 3Ma,representing the formation age of the gabbro pluton. The age is similar to that of the volcanic rocks from the Yishenjilike Formation of the Yili-Central Tianshan,and is slightly younger than the peak metamorphic age of the western Tianshan eclogites. The dating result of the gabbroic rocks indicates that the volcanic activity of the Dahalajunshan Formation ended by the end of the Early Carbonaceous,and the Akeshake Formation sediments were formed between 320 ~ 311 Ma. Magmatic rocks coeval with the gabbroic rocks are widespread within the Yili-Central Tianshan block,and were probably formed in a tectonic setting of transition from compression to extension following the termination of the northward subduction.

关 键 词：辉长岩 Cameca锆石U-Pb年龄 地球化学 大哈拉军山组火山岩 西天山 

分 类 号：P588.124[天文地球—岩石学] P597.3[天文地球—地质学]