甘肃北山地区上石炭统——下二叠统干泉组的时代、分布及其构造意义

Geochronology and Distribution of the Upper Carbonifeous—Lower Permian Ganquan Formation in the Beishan Region,Northwestern China and Its Tectonic Implication

甘肃北山地区位于中亚造山带最南端,上古生界的沉积层序对探讨该区构造与盆地演化有重要意义。上石炭统—下二叠统干泉组的时代和分布尚不明确,限制了沉积层序和构造演化的研究。本研究通过对北山南部独山地区和柳园地区的地层学研究,获得干泉组上段火山岩LA-ICP-MS锆石U-Pb年龄293.8±4.8 Ma和"石炭系未分"火山岩年龄293.9±2.7 Ma和291.7±2.3 Ma。同位素年龄和古生物数据证明干泉组时限为晚石炭世卡西莫夫期(Kasimovian)—早二叠世萨克马尔期(Sakmarian),在研究区广泛出露。北山南部下石炭统维宪阶—中二叠统卡匹敦阶为基本连续的沉积层序,代表了研究区由小型断陷海盆发育为大型裂谷盆地的演化过程:早石炭世维宪期—早二叠世阿瑟尔期,白山组、石板山组和干泉组下段为一系列分散的断陷海盆中的滨浅海相沉积物;早二叠世萨克马尔期(约294~292 Ma),干泉组上段的火山岩表明盆地演化进入火山活动强烈的裂陷期;早二叠世亚丁斯克期—中二叠世卡匹敦期(约286~265 Ma),双堡塘组和菊石滩组组成裂谷盆地沉降期的海侵充填序列。

Objectives：The Beishan Region is located in the southernmost part of the Central Asian Orogenic Belt where the Upper Paleozoic sedimentary sequence plays an important role in the tectonic and sedimentary evolution. However, the age range and distribution of the Ganquan Formation remain uncertain.Methods：Based on sections in the Dushan and Liuyuan areas, southern Beishan region, this study reports LA ICP MS zircon U Pb ages from the Ganquan Formation and ＂undifferentiated Carboniferous＂.Results：The U Pb ages of the Ganquan Formation and ＂undifferentiated Carboniferous＂ are 293.8 ± 4.8 Ma, 293.9 ± 2.7 Ma, 291.7 ± 2.3 Ma, which indicate the Kasimovian-Sakmarian range to the widely distributed Ganquan Formation with paleontological data. The lag deposits of the Shuangbutang Formation aged 293.4 ± 2.0 Ma and 294.4 ± 2.8 Ma suggest an unconformity between the Ganquan and Shuangbutang formations with a six million years sedimentary gap.Conclusions：The Visean-Capitanian strata in the Beishan Region form a generally continuous sequence, which represents the tectonic translation from several small extensional basins into a large rift basin. The Baishan Formation, Shibanshan Formation and the lower part of the Ganquan Formation are considered as the marginal and shallow marine deposits in the separated small basins. The upper part of the Ganquan Formation represents the early synrift phase （ca. 294 ~ 292 Ma） marked by intensive volcanism, whereas the Shuangbutang and Jushitan formations make up a transgression succession the late synrift phase （ca. 286 ~ 265 Ma）.