桂北地区丹洲群锆石U-Pb年代学及对华南新元古代裂谷作用期次的启示

摘要

The Danzhou Group in the northern Guangxi is a suite of typical rift filling successions in the southern Nanhua Rift Basin. For a better understanding of the Neoproterozoic rifting stages in South China, it is important to determine the specific depositional age and regional stratigraphic correlation of the Danzhou Group. In this study, LA-ICP-MS zircon U-Pb dating was carried out on magmatic zircons from the tuff interbeds in the lowermost part of the upper Hetong and Gongdong Formations, and yielded ages of 801±4 Ma (MSWD=0.08) and 781±5 Ma (MSWD=0.04), respectively. Regionally, the Baizhu and the lower Hetong Formations of the Danzhou Group should be correlated with the Jialu and Wuye Formations of the Xiajiang Group, the Cangshuipu and Madiyi Formations of the Banxi Group, the Sunjiahe Formation of the Xixiang Group, and the lower Luliang Formation, which were deposited between 820 Ma and 800 Ma. The upper Hetong Formation should be equivalent to the Fanzhao Formation that was formed between 800 Ma and 780 Ma. The Gongdong Formation should be compared with the Qingshuijiang, Pinglue, and Longli Formations of the Xiajiang Group, the middle-upper Wuqiangxi and Niuguping Formations of the Banxi Group, the middle-upper Dashigou and Sanlangpu Formations of the Xixiang Group, the upper Luliang Formation, and the middle-upper part of Chengjiang, Kaijianqiao, Liantuo, Hongchicun and Shangshu Formations, and their depositional ages were dated to be 780‒ 725 Ma. Representative zircon U-Pb ages from the typical filling successions in the Neoproterozoic rift basins were compiled. It is demonstrated that there are five peak ages such as 818±2 Ma, 802±1 Ma, 780±4 Ma, 756±4 Ma and 728±5 Ma. Among them, 818±2 Ma represents the initial age of the sedimentary overlap in the Neoproterozoic rift basins; 802±1 Ma represents the starting age of drastic basin subsidence together with abundant bimodal magmatism; 780±4 Ma and 756±4 Ma are the two peak ages of magmatic intrusions, however, the sedimentation had no apparent transitions; 728±5 Ma approached the top age of the pre-Sturtian successions, suggesting the onset of Sturtian glaciation in South China should be later than ca. 720 Ma, rather than 780 Ma. Combined with sediment geological data, we further suggest that the Neoproterozoic rifting in South China can be divided into two stages, one was 820‒ 800 Ma, and the other 800‒ 725 Ma. Moreover, there is obvious coupling relationships between the Neoproterozoic magmatism and rifting in South China; nevertheless, even though the impact of episodic magmatism in each basin varied.%桂北地区丹洲群是南华裂谷盆地南段的一套连续裂谷充填沉积，厘定各组沉积时限及区域地层关系，对理解华南新元古代裂谷作用期次具有重要意义。本文利用LA-ICP-MS锆石U-Pb同位素测年，获得丹洲群合桐组二段和拱洞组底部凝灰岩夹层形成年龄分别为801±4 Ma和781±5 Ma。研究表明，丹洲群白竹组和合桐组一段与下江群甲路组和乌叶组、板溪群沧水铺组和马底驿组、西乡群孙家河组及陆良组一段相当，沉积时限为820~800 Ma；合桐组二段与下江群番召组相当，沉积时限为800~780 Ma；拱洞组可与下江群清水江组、平略组和隆里组，板溪群五强溪组中上部和牛牯坪组，西乡群大石沟组中上部和三郎铺组，陆良组二段及澄江组、开建桥组、莲沱组、虹赤村组和上墅组的中上部直接对比，沉积时限为780~725 Ma。华南新元古代裂谷盆地系统的典型地层锆石年龄存在5组高峰，峰值年龄分别为818±2 Ma、802±1 Ma、780±4 Ma、756±4 Ma及728±5 Ma。综合华南新元古代岩浆活动特征及盆地沉积演化过程，确定华南新元古代裂谷作用可分为两期：820~800 Ma和800~725 Ma。此外，华南新元古代岩浆活动与裂谷作用之间存在明显的耦合关系，但各期岩浆活动对各裂谷盆地的影响程度存在差异。