新疆白干湖地区花岗岩年代学、地球化学研究及其构造意义

Geochronology,geochemistry and tectonic significance of granites in Baiganhu area,Xinjiang

新疆白干湖地区处于东昆仑-祁漫塔格构造带与阿尔金地块的结合部位，出露的巴什尔希岩浆序列对理解该区早中志留世的构造环境有重要意义。本文在巴什尔希岩浆序列中白干湖岩体的不同部位采集了二长花岗岩（BGH-1）和正长花岗岩（BGH-4）两件样品，并进行了详细的岩相学、LA-ICP-MS锆石U-Pb年代学和地球化学研究。白干湖岩体含有白云母、石榴子石等特征矿物，其A／CNK和A／NK的分布范围分别是1．07～1．12和1．41～1．59，属于弱过铝质高钾钙碱性S型花岗岩，并具有富集LREE和LILEs、亏损HFSEs，Nb、P、Ti明显负异常的特点。锆石U-Pb定年结果显示，二长花岗岩形成于428．2±4．2Ma，正长花岗岩形成于422．5±2．3Ma。本文和前人的研究成果表明，巴什尔希岩浆序列具有S型花岗岩与A型花岗岩共生的特点，主要活动时代为早中志留世（433～421Ma），形成于后碰撞伸展构造体制。然而，在白干湖断裂以东至格尔木的东祁漫塔格，早中古生代岩浆活动记录了晚奥陶世一早志留世俯冲、中志留世一早泥盆世碰撞和早泥盆世晚期开始后碰撞的演化过程，其中东祁漫塔格早中志留世岩浆岩和白干湖断裂西侧的巴什尔希岩浆序列分别形成于不同的构造环境，因此二者很可能属于不同的构造体系。

Baiganhu area is situated between East Kunlun-Qimantag tectonic zone and Altun block, where the Bashierxi granite series is the key to understanding the tectonic setting of this area during the Early-Middle Silurian. In this paper, a monzonitic granite （BGH-1） and a syenogranite （BGH-4） samples were collected from a single pluton and used for petrographic observations as well as geochronological and geochemical studies. This pluton contains muscovite and garnet, and its A/CNK and A/NK ratios are in the ranges of 1.07～1.12 and 1.41～1.59, respectively. Thus this pluton is peraluminous and high-K calcalkaline Stype granite. In addition, this pluton is characterized by enrichment of LREE and LILEs and depletion of HFSEs, with obvious negative anomalies of Nb, P and Ti. LA-ICP-MS zircon U-Pb dating reveals that the monzonitic granite and syenogranite were emplaced at 428.2± 4.2 Ma and 422.5 ±2.3 Ma, respectively. The data obtained by the authors and previously published data show that the calcalkaline Stype granites and Atype granites are coexisting in Bashierxi granite series and they were formed in a post-collisional environment during the Early-Middle Silurian （433～421 Ma）. However, the Early-Middle Paleozoic magmatism in eastern Qimantag between Golmud and Baiganhu fault to the east of the Baiganhu fault has recorded a complete process of Late Ordovician to Early Silurian subduction, Middle Silurian to Early Devonian collision, and late Early Devonian postcollision. Apparently, the Early-Middle Silurian magmatic plutons in eastern Qimantag and the Bashierxi granite series, which are outcropped in Baiganhu area to the west of the Baiganhu fault, were formed in different tectonic environments, thus probably belonging to different tectonic evolution systems.