塔里木盆地西北缘乌什地区上寒武统—下志留统碎屑锆石U-Pb年代学及对物源体系和构造演化的指示

Detrital zircon U-Pb geochronology of Upper Cambrian-Lower Silurian sandstone in the Wushi area, northwestern margin of Tarim basin:implications for provenance system and tectonic evolution

早古生代塔里木盆地构造背景发生急剧突变的地球动力学来源一直是一个争议的焦点。本文针对塔里木盆地西北缘乌什地区寒武系-志留系剖面的砂岩样品进行了碎屑锆石U-Pb年代学测试,以此厘定研究区物源体系并刻画该地区的沉积构造演化过程。结果表明,碎屑锆石样品记录了~500Ma、~800Ma、~1000Ma、~1800Ma、~2500Ma共5期构造热事件。对比潜在物源区,上寒武统物源主要为盆地内碎屑物的再循环沉积,直接来源于柯坪或邻近地区的局部隆起,最终来源于盆地内更古老岩石的风化剥蚀。至早志留世,物源区则可能增加了昆仑造山带,同时也说明早志留世西昆仑洋已经俯冲闭合,甚至隆升形成高山。综合前人研究结果,晚奥陶世塔里木盆地构造背景突变、台地消亡的原因,可能主要来自于盆地南侧的洋盆的俯冲闭合造山,而是否有来自北侧洋盆的影响还需要进一步的工作,但即使有来自北侧洋盆活动的影响,其力度和范围也远小于南边。

The geodynamic origin of the abrupt change in the tectonic setting of the Tarim basin in the Early Paleozoic has long been the focus of controversy. In this paper, detrital zircon U-Pb geochronology was performed on sandstone samples from the Cambrian-Silurian section in the Wushi area, northwest margin of Tarim basin, to determine the provenance system and describe the sedimentary tectonic evolution in the area. The results show that the detrial zircon samples record five tectonic thermal events occuring in ~ 500 Ma, ~ 800 Ma, ~ 1000 Ma, ~ 1800 Ma, and ~ 2500 Ma. Compared with the potential sources, the provenance of the Upper Cambrian probably resulted from regional uplifting in Keping or nearby areas, as well as weathered errosive ancient rocks within the basin. By the Early Silurian, the Kunlun orogenic belt had become one of the provenances of the study area, and also indicates that the Kunlun Ocean has been subducted and closed and even uplifted to form high mountains in the Early Silurian. According to the previous studies, it can be concluded that the reason for the abrupt change of tectonic background and the disappearance of the platform in the Tarim basin in the Late Ordovician may derive from the subduction and closure of the ocean basin in the south of the basin. Moreover, some further work needs to be carried out to decide whether there was the involvement of oceanic basin in the north. However, although there is some influence from the oceanic basin, its impact and scope should be far less than that in the south.