新疆博格达山晚古生代地层的形成时代、物源及其演化:碎屑锆石U-Pb年代学证据

新疆博格达山主体由石炭系海相火山—沉积岩系组成, 以发育两期双峰式火山岩, 但不发育花岗岩为特征, 对其晚古生代地层时代的划分和演化争议较大.本文重点对博格达山北部两个晚古生代砂岩进行了碎屑锆石U-Pb年代学分析, 重新标定博格达山地区晚古生代地层的形成时代; 利用物源区的演化, 约束晚古生代构造演化.测年结果显示博格达上亚群砂岩的碎屑锆石表面年龄值分布范围较宽, 主峰年龄为343～284 Ma(80%), 次峰年龄为386～375 Ma(3%)、503～441 Ma(7%)和871～735 Ma(10%); 芦草沟组砂岩的碎屑锆石表面年龄值非常集中, 主峰年龄为358～279 Ma(97%), 次峰年龄为257～251 Ma(约3%).博格达山中部原石炭纪博格达群上亚群与西部和南部下芨芨槽群相当, 应属于早二叠世, 中部—东部的石炭—二叠纪界线应在博格达下亚群—上亚群或居里得能组—沙雷塞尔克组之间的不整合面之中.博格达北部地区晚二叠世以南侧天山物源区供给为主, 反映出晚古生代期间博格达山地区至少存在晚石炭世末和中二叠世两期构造隆升.结合区域火山岩与火山碎屑岩的研究, 认为博格达山地区晚古生代主要经历4个演化阶段: 早石炭世弧后盆地裂解阶段、晚石炭世碰撞拼贴阶段、早二叠世碰撞后伸展阶段、中-晚二叠世再次隆升到稳定阶段.

Age,provenance and tectonic evolution of Late Paleozoic strata in Bogda Mountain,Xinjiang: Evidence from detrital zircon U-Pb geochronology

There were quite different opinions on age of Late Paleozoic strata and its tectonic evolution in Bogda Mountain, Xinjiang, which mainly consist of Carboniferous marine volcanic-sedimentary rocks including two periods of bimodal volcanic rocks but little granites. In this paper, age recalibration and provenance analysis of Late Paleozoic strata were done by LA-ICPMS U-Pb isotope dating of detrital zircons of two sandstones from Late Paleozoic strata in northern Bogda Mountain with published zircon U-Pb age of magmatic rocks, as well as Late Paleozoic tectonic evolution constrained by variation of detrital provenances was offered. The highest peak of wide detrital zircon U-Pb age range of sandstone sample from upper subgroup of Bogda Group was between 343～284 Ma(80%)with minor peaks between 386 Ma and 375 Ma(3%), 503 Ma and 441 Ma(7%), and 871 Ma and 735 Ma(10%). While the age of sandstone sample from Lucaogou Formation was highly concentrated between 358 Ma and 279 Ma(97%)with 257 Ma and 251 Ma(3%). Upper subgroup of Bogda Group should belongs to Early Permian rather than Late Carboniferous, and be equivalent with Lower Jijicao Group in southern Bogda Mountain. The Permo-Carboniferous boundary was suggested to be unconformity between lower and upper subgroup of Bogda Group or Julideneng and Shaleikesaier Formation. The detrial material of northern Bogda area during Permian was from northern and southern binary provenances, therein the mainnest provenance was the northern during Early Permian while the southern during Late Permian. The variation of detrial provenances indicated two periods of tectonic uplift during Late Carboniferous and Middle Permian. Refering to research on volcanic and volcaniclastic rocks, the tectonic evolution during Late Paleozoic era in Bogda Mountain was divided into four stages: back-arc rift basin during Early Carboniferous, collision during Late Carboniferous, post-collisional extension during Early Permian and uplift again to stable during Middle to Late Permian.