新疆萨吾尔地区二叠纪火山岩地球动力学背景

Geodynamic background of Permian volcanic rocks in the Sawuer region,Xinjiang

研究区位于新疆维吾尔自治区吉木乃县境内,地处萨吾尔山中段,属于哈萨克斯坦-准噶尔板块北缘。区内发育一套卡拉岗组和哈尔加乌组火山岩,时代为早二叠世。岩性主要为以玄武岩、玄武质安山岩、钾玄岩、粗安岩和粗面岩为代表的基性-中偏碱性岩石组合,化学成分上具有富碱、富钾、贫钛等特点,均显示"双峰式"火山岩特征;岩石的大离子亲石元素如Rb、Ba、Th、Pb等相对富集,Ta、Nb和Ti呈明显的"TNT"负异常,Sr、P、Ti、Be、Cu、Bi等相对亏损,而Rb、La、Nd等则相对富集,显示后碰撞火山岩的稀土元素和微量元素特点,结合火山岩中玄武岩构造环境判别图解,推断当时该区处于主碰撞之后的拉张伸展地球动力学背景。

The Permian volcanic rocks in the Sawuer region is situated in the middle part of Sawuer Mountain,Jimunai County of Altay District,Xinjiang. The region belongs to the north margin of the Kazakhstan-Junggar plate. In the study area, there occurs a set of volcanic rocks of Kalagang Formation and Haerjiawu Formation,the epoch of which is early Permian. These rocks are mainly a set of basic to intermediate to alkali rock association composed predominantly of basalt, basaltic candesite, shoshonite,trachyandesite, and trachyte. Chemically they are characterized by enrichment in alkaline and potassium and poverty in titanium. The volcanic rocks of the two formations show ＂bimodal＂characteristic. These rocks have rich LILE,such as Rb,Ba,Th, and Pb,and Ta, Ti and Nb in the rocks show obviously negative ＂TNT＂anomalies. Such elements as Rb, La and Nd in these rocks are enriched relatively in comparison with the ones such as Sr,P,Ti,Be,Cu, and Bi. These results show the characteristics of trace elements and REE of post-collisions. Based on the diagrams of the tectonic setting of basalt in the volcanic rocks, it is inferred that the region geodynamically belong to the extensional tectonic setting of post collision after the major collision.