新疆准噶尔古生代洋盆闭合时限——来自卡拉麦里地区石炭纪碎屑锆石U-Pb年代学的约束

新疆准噶尔古生代洋盆闭合时限对中亚造山带古生代构造格局及演化研究具有重要意义。东准噶尔卡拉麦里断裂带南缘广泛出露石炭纪陆相粗碎屑岩系,沉积相分析表明其形成于扇三角洲沉积环境。依据区域地层对比、岩石组合特征及地层接触关系,将其重新厘定为山梁砾石组。选择西段滴水泉和东段双井子2个地区的山梁砾石组剖面进行地层对比,并在2个剖面底部采集粗砂岩样品进行LA-ICP-MS锆石U-Pb年龄测定,获得最年轻的碎屑锆石年龄分别为349±4Ma和355±3Ma,代表山梁砾石组沉积时代晚于349Ma,应为早石炭世早期。碎屑锆石年龄分布特征及砾石成分表明,其物源主要来自断裂带北侧的泥盆纪火山岩。在分析前人资料的基础上,认为山梁砾石组碎屑岩系是卡拉麦里造山带强烈隆升造山过程的沉积学响应,形成于前陆盆地,限定了准噶尔古生代洋盆闭合时限在早石炭世早期之前。

The closure time of Junggar Paleozoic oceanic basin: Evidence from Carboniferous detrital zircon U-Pb geochronology in Kalamaili area

The closure time of Junggar Paleozoic oceanic basin is significant for the reconstruction of Paleozoic tectonic framework and evolution of the Central Asian Orogenic Belt (CAOB). The Carboniferous coarse clastic rocks, which are widely distributed in the south of the Kalamaili fault belt, were formed in a fan-delta environment by sedimentary facies analysis. On the basis of regional stratigraphic correlation, rock association characteristics and stratigraphic contact relations, the authors redefined the coarse clastic rocks as the Shanlianglishi Formation. Two profiles of Shanlianglishi Formation from Dishuiquan and Shuangjingzi area, which were respectively located in the western and eastern section of Kalamaili fault belt, were chosen to correlate the stratigraphic sequence. UPb ages of detrital zircons from two coarse sandstone samples on the bottom of Shanlianglishi Formation were measured using the LA-ICP-MS method. The ages of the youngest zircons, 349±4Ma and 355±3Ma, indicate that the sedimentary age of Shanlianglishi Formation were later than 349Ma, and it might have been formed in the early Early Carboniferous. The ages of detrital zircons and the gravel composition show that the provenance of Shanlianglishi Formation was mainly derived from the Devonian volcanic rocks in the north of the Kalamaili fault belt. Integrating the previous data, the authors infer that the coarse clastic rocks formed in the foreland basin were probably the sedimentary response to the uplifting orogenic process of the Kalamaili orogenic belt, and the closure time of Junggar Paleozoic oceanic basin is constrained before the Early Carboniferous.