南天山北部巴音布鲁克早古生代火成岩的地球化学特征与岩石成因  

Geochemical characteristics and petrogenesis of the Early Paleozoic igneous rocks from Bayinbuluke,Southern Tianshan

作　　者：马中平[1] 夏林圻[1] 徐学义[1] 夏祖春[1] 李向民[1] 王立社[1] 

机构地区：[1]西安地质矿产研究所,西安710054

出　　处：《岩石学报》2008年第10期2289-2300,共12页Acta Petrologica Sinica

基　　金：中国地质调查局项目(1212010611804;200313000063);国家自然科学基金项目(40472044)资助

摘　　要：南天山北部上志留统巴音布鲁克组火山岩系以中基性喷出岩为主,并有辉长岩侵入,其矿物组合中都含有角闪石,岩层中爆发相火山碎屑岩极为发育。各种火山—侵入岩的LREE和 LILE强烈富集、HFSE相对亏损,出现明显的Ta、Nb负异常,(La/Yb)_N=2.92～5.21,Nb/La=0.22～0.33,具有岛弧岩浆岩的特征。Zr/Nb=26～47、Hf/Ta=11.9～19.9,暗示其源区为类似于N-MORB的亏损地幔;高的Th、LREE含量和高于正常弧火山岩、低于洋岛玄武岩(OIB)的HFSE绝对丰度,较小的ε_(Nd)(t)值(+3.23～+6.15)、较大的(^(87)Sr/^(86)Sr)t值(0.7044～0.7063),ε_(Nd)(t)与(^(87)Sr/^(86)Sr)_1间呈明显的线性负相关关系,并且分别与LILE/HFSE之间呈负相关和正相关关系。上述特征反映出巴音布鲁克火成岩的岩石成因中既有俯冲洋壳板片流体—沉积物的加入,也有板片熔体组分的贡献,在地球化学上具有正常岛弧玄武岩(IAB)与富铌岛弧玄武岩(NEAB)之间过渡系列的特征。该套火成岩与中天山南缘蛇绿混杂岩带紧邻,地层时代也与南天山洋俯冲—消减时代相当,它应当是南天山洋盆于晚志留世时发生的俯冲—消减所引发的岛弧火山—岩浆作用的地质记录。The volcanics of Upper-Silurian Bayinbuluke Formation in the north of Southern Tianshan are mostly composed of intermidiate-basic extrusive rocks with gabbro intrusion and abundant pyroclastic rocks. These volcanics and gabbros are enriched in LILEs and LREEs with （La/Yb）N values 2. 92 - 5.21, and depleted in HFSEs, especially Nb and Ta negative anomaly with Nb/La values 0. 22 -0. 33, similar to the magmatie rock in island arc settings. The Zr/Nb ratios （26 - 47 ） and Hf/Ta ratios （11.9 - 19. 9） indicate their sources are depleted mantle sources similar to N-MORB. The higher contents of HFSEs than typical island arc basalt （IAB） and the linear negative correlation between 650 （t） （ ＋ 3.23 - ＋ 6. 15） and （^87Sr/^86Sr）t values （0. 7044 - 0. 7063） which display negative and positive correlation with LILE/HFSE, respectively, suggest that both fluid-sediments and silicate melts derived from subducted oceanic slab are involved in the genesis of Bayinbuluke magmas, and these rocks are the intergradation between normal island arc basalt （IAB） and Nb-enriched arc basalt （NEAB）. According to the distributing location of Bayinbuluke Formation adjacent to the ophiolitic melange belt on the southern margin of Central Tianshan and its chronologic age corresponding with the subducting age of Southern Tianshan Ocean, it stands a good chance that the formation of these volcanics and gabbros owes to the Late-Silurian subduction of Southern Tianshan Ocean.

关 键 词：火成岩 地球化学 岩石成因 上志留统 巴音布鲁克组 南天山 

分 类 号：P588.125[天文地球—岩石学]