中亚造山带南缘晚石炭世——早二叠世碳酸盐岩台地的建立和消亡

Formation and Demise of Late Carboniferous to Early Permian Carbonate Platforms on the Southern Margin of the Central Asian Orogenic Belt

碳酸盐岩台地具有重要的古海洋环境指示意义。中亚造山带南缘在晚石炭世—早二叠世出现大规模分布的碳酸盐岩地层,但这些岩石地层单元缺少精确时代约束和系统对比研究,因而制约了进一步古地理和古环境意义的讨论。此次研究报道了阿拉善地块北缘“阿木山组”实测剖面的地层层序,提供了火山岩夹层的锆石U-Pb同位素年龄,精确约束了碳酸盐岩台地的时代和分布。研究表明:研究区恩格尔乌苏断裂带以北的“阿木山组”应修订为“格根敖包组”,以碎屑岩为主,时代为晚石炭世卡西莫夫期—早二叠世阿瑟尔期(<296.1 Ma)。该断裂带以南的阿木山组的碳酸盐岩台地以连续出露、富含?类化石的灰岩为标志,时代为晚石炭世格舍尔期—早二叠世阿瑟尔期(303.2~295.3 Ma)。笔者根据研究区与甘肃北山、内蒙古中部等地区的高精度地层对比提出,中亚造山带南缘的碳酸盐岩台地是全球晚石炭世海侵和古亚洲洋南缘亚热带气候的产物,早—中二叠世的裂谷作用是这些碳酸盐台地消亡的主要原因。

Carbonate platforms have significant implications for the ancient marine environment.During the Late Carboniferous to Early Permian,continuous deposited carbonate strata were widely distributed on the southern margin of the Central Asian Orogenic Belt.However,these rock units lack precise age constraints and systematic correlation,thus limiting further discussions on their paleogeographic and paleoenvironmental significance.This study reports the stratigraphic sequence of the"Amushan Formation"measured sections on the northern margin of the Alxa Block and provides zircon U-Pb isotope ages from volcanic intercalations,precisely constraining the age and distribution of the carbonate platforms.The results indicate that the"Amushan Formation"north of the Engerwusu Fault Zone should be revised to the"Gegenaobao Formation,"which is dominated by clastic rocks and dates to the Kasimovian to Asselian(<296.1 Ma).South of the fault zone,the carbonate platform of the Amushan Formation is characterized by continuously deposited and fusulinid-rich limestone with ages from the Gzhelian to Asselian(303.2~295.3 Ma).Based on high-resolution stratigraphic correlations with areas such as Beishan in Gansu and central Inner Mongolia,this study proposes that the carbonate platforms at the southern margin of the Central Asian Orogenic Belt were a product of global Late Carboniferous transgression and the subtropical climate of the southern margin of the Paleo-Asian Ocean.The rifting during the Early to Middle Permian was the primary reason for the demise of these carbonate platforms.