Permian mountain building in the bogda mountains of NW China

The West Bogda Mountains separate the two largest petroliferous sedimentary basins in Northwest (NW) China and may record the evolution history of the Central Asian Orogenic Belt (CAOB). The timing of the initial uplift of the West Bogda Mountains is debated, which may provide the robust constraint on the CAOB evolution. Here we present an integrated study of sedimentology, geochemistry, and detrital zircon geochronology to constrain the timing of the initial uplift of the West Bogda Mountains. The late Lower Permian Lucaogou Formation in the West Bogda Mountains is dominated by organic-rich laminated shale, gravity flow deposits and dolomite without shallow-water sedimentary structures, suggesting a deep-water lacustrine depositional environment. The Middle Permian Quanzijie Formation is mainly composed of poorly sorted and sub-rounded conglomerate, which indicates that the depositional environment changed to an alluvial fan. The depositional environment changed to a meandering river system in the early Upper Permian Wutonggou Formation, with evidence from the widely developed cross-bedding in its sandstone and the typical ‘dual structure’. The compositions of immobile elements and detrital zircon U-Pb geochronology reveal changes in sediment sources from the early to late Permian. The sediments of the late Lower Permian Lucaogou Formation were mainly sourced from the North Tianshan, while the sediments in the Middle Permian were mostly locally derived. Combining these evidences, we infer that the initial uplift of the West Bogda Mountains likely occurred in the period from the late Lower to Middle Permian (~289.8 Ma to ~265.7 Ma), and was probably caused by the intracontinental collision between Junggar Block in the north and the Tarim Block in the south.

西博格达山脉分隔中国西北两大含油气沉积盆地，且可能记录了中央造山带（Central Asian Orogenic Belt，CAOB）的演化历史。目前学界对西博格达山脉的初始隆升时间尚存争议，而该时间可为中央造山带的演化提供强有力的约束。本文综合运用沉积学、地球化学以及碎屑锆石年代学方法，对西博格达山脉的初始隆升时间进行约束。西博格达山脉地区下二叠统芦草沟组（Lucaogou Formation）上段以富有机质纹层页岩、重力流沉积以及缺乏浅水沉积构造的白云岩为主，指示深水湖泊沉积环境。中二叠统泉子街组（Quanzijie Formation）主要由分选差、次圆状砾岩组成，表明沉积环境转变为冲积扇。上二叠统梧桐沟组（Wutonggou Formation）下段的沉积环境转变为曲流河体系，其依据为砂岩中广泛发育的交错层理以及典型的“二元结构”。不活动元素组成与碎屑锆石U-Pb年代学结果显示，二叠纪早晚期的沉积物源区发生了变化。下二叠统芦草沟组上段的沉积物主要物源为北天山，而中二叠统沉积物则多为近源沉积。综合上述证据，本文推断西博格达山脉的初始隆升可能发生于下二叠统晚期至中二叠世（约289.8 Ma至265.7 Ma），其成因可能为北部准噶尔地块（Junggar Block）与南部塔里木地块（Tarim Block）之间的陆内碰撞。