科克苏-穹库什太古生代构造-岩浆作用及其对西南天山造山时代的约束

Paleozoic tectonism and magmatism of Kekesu-Qiongkushitai section in southwestern Chinese Tianshan and their constraints on the age of the orogeny

天山造山带是古生代多期碰撞增生作用的产物,其确切的造山时代是当前争议较多的热点问题。分布在西南天山的科克苏-穹库什太剖面经历了复杂的构造变形,最明显的两期变形事件分别为朝北的推覆作用和 NE-SW 韧性走滑作用。本剖面可分为伊犁岩浆岛弧、伊犁结晶基底、高压变质杂岩三个岩石-构造单元。野外可见黑云母花岗闪长岩侵入到绿片岩相变质岩中,岩脉切穿绿片岩中面理构造。通过锆石 U-Pb LA-ICPMS 测年,科克苏剖面钾长花岗岩的年龄为341±6Ma 和338±8Ma,穹库什太黑云母花岗闪长岩的年龄为313±4Ma。其中,花岗闪长岩发生了黑云母定向排列,通过对黑云母进行^(40)Ar/^(39)Ar 测年,获得坪年龄为263.4±0.6Ma,表明该花岗闪长岩受过后期热事件的干扰。地球化学分析表明,黑云母花岗闪长岩属于钙碱性系列,Nb 和 Ta 含量低而 Rb,Ba 和 Th 含量很高,与俯冲作用有关的岛弧岩浆岩地球化学组成非常相似。结合前人对该地区高压变质岩、花岗岩和火山岩的研究成果,本文提出,西南天山俯冲-碰撞造山作用发生在晚石炭世之前,研究区后碰撞区域走滑作用标志着西南天山碰撞造山作用在二叠纪之前全部结束。

The Tianshan is a polyphase collision-accretion orogenic belt, its ages of orogeny is an issue in hot debate. The Kekesu- Qiongkushitai section of southwestern Chinese Tianshan experienced complicated deformations including mainly top-to-the-north thrust and NE-SW extending ductile strike-slip faulting. This section can be subdivided into three lithotectonic units, namely Yili magmatic arc, Yili Proterozoic basement and HP metamorphic complex. In the field, granodiorite of Yili arc intrudes in greenschist facies metamorphic rocks cutting foliations of greenschists. Zircon U-Pb LA-ICPMS dating yields 341 ± 6 kla and 338 ±8Ma for K-granite from Kekesu River, and 313 ± 4Ma for biotite-beating granodiorite from Qiongkushitai, respectively. Biotite from granodiorite yields a ^40 Ar/^39 Ar plateau age at 263.4 ± 0.6Ma, revealing an important thermal overprint in Permian. Geochemistry of granodiorite indicates that it belongs to calc-alkaline series, with significant low compositions in Nb and Ta, and enrichment in Rb, Ba and Th. These features are consitent with the geochemical components of arc-type magmatic rocks. Taking into account the previous results on HP metamorphic and magmatic rocks in west Tianshan areas, this paper proposes that the subduction-collision events in southwestern Chinese Tianshan occurred before Late Carboniferous, post-collisional ductile strike-slip faulting confirms that the collisional orogenic processes ended in Permian for the southwestern Chinese Tianshan belts.