天山古生代增生造山作用及其构造转换事件

Paleozoic accretionary orogenesis and major transitional tectonic events of the Tianshan orogen

天山造山带自新元古代以来,经历了漫长而复杂的俯冲增生造山作用和陆内构造活化过程,属于典型的复合型造山带。基于近年研究进展,本文对伊犁、境内外中天山和南天山构造带前寒武纪基底、古生代沉积序列、多期陆缘弧岩浆岩和构造缝合带的变形变质特征、形成环境和年代学等进行了总结分析,梳理了天山古生代增生造山作用中的三次重要构造转换事件及其地质记录。(1)伊犁南北两缘、中天山、南天山和塔里木北缘,均发育中奥陶世—志留纪的大陆弧岩浆作用,伊犁北缘、南天山-塔里木北部早古生代沉积环境发生显著变化,表明天山-塔里木北缘在中—晚奥陶世发生了从被动陆缘向活动陆缘的转换。(2)伊犁南、北两缘和中天山的早古生代岩层在晚志留世—早泥盆世普遍发生了强烈的韧性变形和角闪岩相变质作用,其上不整合覆盖有弱变形未变质的晚泥盆世—石炭纪火山-沉积地层;该区域不整合是哈萨克斯坦微大陆拼合事件在研究区的构造响应,也标志着准噶尔洋和南天山洋的俯冲方式在泥盆纪发生了由前进式(东太平洋型)向后撤式(西太平洋型)的构造转换,导致伊犁和中天山在晚泥盆世—石炭纪经历了伸展背景下的大陆弧岩浆作用,在南天山-塔里木北缘则形成了一系列弧后有限洋盆。(3)天山各构造单元及其边界缝合带中普遍发育晚石炭世逆冲推覆构造和二叠纪走滑韧性剪切带、晚石炭世—早二叠世滑塌堆积和二叠纪后造山岩浆岩,指示晚石炭世—早二叠世发生了由汇聚造山向陆内构造的转换。这些构造转换事件是认识古亚洲洋各分支洋盆从初始俯冲、俯冲方式转换到俯冲终结过程的基础,也是探讨增生造山动力学的关键。

The Tianshan orogen underwent long-lived subduction/accretionary orogenesis and complicated intracontinental reactivation since Neoproterozoic,it is a typical composite orogenic belt.Based on the recent research progress,this article analyzes available structural,metamorphic and chronological data of the Precambrian basement,Paleozoic sedimentary sequences,arc-type magmatic rocks in the Yili,Central Tianshan and South Tianshan units,as well as the deformed and metamorphic rocks along the major suture zones,and summarizes geological records for three major transitional tectonic events.(1)The first transitional event from passive to active margins is suggested by the Middle to Late Ordovician continental arc magmatism and changes of sedimentary environments in northern and southern Yili block,Central Tianshan and northern Tarim.(2)The second transitional event from advancing(Eastern Pacific-type)to retreating(Western Pacific-type)subduction regimes is proposed on the basis of Late Silurian to Early Devonian intensive ductile deformation and amphibolite facies metamorphism recorded in the Early Paleozoic rocks of the Yili and Central Tianshan blocks,this event was also a response to the amalgamation of the Kazakhstan microcontinent;subsequently,continental arc magmatism under extensional setting related to retreating subduction regime occurred in the Yili and Central Tianshan blocks during Late Devonican to Carboniferous,and a series of back-arc limited oceanic basins formed simultaneously in South Tianshan.(3)The third transitional event from convergent orogeny to post-orogenic intracontinental tectonics is evidenced by widely developed late Carboniferous thrusting and Permian strike-slip shearing,latest Carboniferous to Early Permian olistostrome,and Early Permian post-collisional magmatism.Deciphering these transitional tectonic events is critical in understanding the orogenic processes and geodynamics of the oceanic basins within the Paleo-Asian Ocean from subduction initiation,switching of subduction regimes and subduction termination.