新疆西准噶尔北部早古生代增生造山过程研究

Most studies are recently focused on the geological significance of Silurian magmatism in the Xiemisitai Mountains, with attempts to reveal the tectonic evolution of the northern West Junggar in Xinjiang during the Early Paleozoic. However, more ancient magmatism and essentials of Early Paleozoic geological units to the north are poorly constrained, leading to an uncompleted understanding of dynamic evolution of accretionary orogeny in northern West Junggar. In this project, on the base of detailed investigations in the field, comprehensive analysis methods (such as zircon U-Pb and 40Ar-39Ar dating, whole rock major and trace elements and Sr-Nd-Os isotopes analysis) would be conducted to confirm the ages, geochemistry and tectonic setting of Early Paleozoic volcanic rocks in the Tarbagtay-Saur Mountains; and to constrain the origin of the Hebukesaier ophiolite and their formation and emplacement ages; and to understand the characteristics of volcanic and sedimentary formations, subduction process and duration in the Xiemisitai-Shaerbuerti Mountains. We will illustrate the Early Paleozoic accretionary history across the northern West Junggar by full recognition on the features of individual tectonic units, which would provide key sights on the accretionary orogenic process of the West Junggar during the Early Paleozoic.

围绕新疆西准噶尔北部早古生代如何演化这一重要科学问题，人们目前多集中讨论谢米斯台山志留纪岩浆作用的地质意义，而关于更古老岩浆活动以及北侧若干重要早古生代地质体基本特征与演化的研究很少，导致对该区早古生代增生造山过程认识并不完整。本项目拟在详细的野外调查基础上，综合应用多种测试方法（如：锆石U-Pb、40Ar-39Ar测年、全岩主量、微量元素与Sr-Nd-Os同位素测试等），准确厘定塔尔巴哈台山—萨吾尔山早古生代火山岩的时代、性质与形成背景；限定和布克赛尔蛇绿岩的类型、成因、形成与就位时代；确定谢米斯台山—沙尔布尔提山早古生代火山—沉积建造特征、俯冲作用过程与持续时限。在深入了解以上早古生代构造单元各自的基本组成与演化特征基础上，完成与邻区构造单元的对比、连接，建立西准噶尔北部早古生代相对完整的构造演化模式，加深对西准噶尔古生代增生造山过程的理解。

西准噶尔北部早古生代构造格局与演化历史对于深入理解中亚造山带增生造山过程具有重要科学意义。本项目对西准噶尔北部代表性早古生代地质单元进行了详细的野外地质调查与室内综合分析测试研究，获得若干重要数据资料，取得的主要成果包括：（1）塔尔巴哈台山-萨吾尔山地区原先认为的部分泥盆纪地层实际形成时代跨越志留纪-二叠纪，而且不同时代岩浆分布未表现明显的单向变年轻趋势。该区新发现431Ma-426Ma形成的高镁安山岩与A2型花岗岩均具有亏损地幔的Sr-Nd-Hf同位素特征，形成于西准噶尔北部志留纪弧-弧拼贴之后的局部伸展环境。（2）和布克赛尔蛇绿岩包含相对完整的古大洋残片组分，与古海山、碎屑岩等一起构成典型的蛇绿混杂岩。不同蛇绿岩组分的单矿物、全岩地球化学与Os同位素组成具有弧前SSZ型蛇绿岩特征。蛇绿混杂岩中的辉长岩团块年龄为484 Ma,表明西准噶尔北部博什库尔-成吉斯弧与塔尔巴哈台弧之间发育早奥陶世的古洋盆。后期侵入和布克赛尔蛇绿混杂岩中的岩体年龄为441Ma-435Ma，限定了该洋盆关闭的时间上限。（3）查干陶勒盖蛇绿混杂岩中包含古海山残片组分，代表古海山组分的洋岛型基性岩年龄为469Ma,晚于蛇绿岩中的辉长岩年龄（517Ma）。同时，该蛇绿岩混杂岩中发育500Ma左右的中-酸性岩体，具有初始俯冲岩浆的地球化学特征，暗示准噶尔洋的北向俯冲可能开始于寒武纪。（4）博什库尔-成吉斯弧西段的谢米斯台山以大量发育火山岩、侵入岩为特征，存在540Ma和445-429Ma两期与俯冲作用相关的岩浆活动。与之相比，东段沙尔布尔提山以发育火山碎屑岩-沉积岩地层为主，泥盆系和布克赛尔组砂岩碎屑锆石最大沉积年龄为406Ma-402Ma,年龄谱具有比较集中的单峰年龄（424Ma），表明其可能沉积于弧前环境。相关成果已发表在《Lithos》等期刊上，也为后续研究奠定了坚实的基础。