【摘要】 本文通过对江南雪峰沧水铺地区元古代砾岩及其上下层位的详细野外地质调查及相关样品的锆石U-Pb激光和SIMS年代学研究表明,江南雪峰地区新元古代地层自下到上包括冷家溪群、由林家湾组紫红色砾岩、银珠坝组陆相火山岩(或宝林冲组下部火山岩)构成的沧水铺群、及以横路冲组砂砾岩层开始的板溪群。下伏于银珠坝组陆相火山岩并角度不整合上覆于冷家溪群的紫红色砾岩代表了碰撞造山作用的结束,而上覆于该火山岩系并整合于板溪群马底驿组之下的横路冲组砾岩层则代表了板溪裂谷作用的开始。年代学测试表明下伏于林家湾组紫红色砾岩的冷家溪群顶部砂岩中年轻碎屑锆石集中给出了864Ma的年龄峰值,代表了冷家溪群的最大沉积顶界年龄;而上覆于林家湾组紫红色砾岩的银珠坝组火山岩、或称之为宝林冲组下部火山岩、给出了(835±12)Ma的SIMS206Pb/238U加权平均值,上述年龄界定了林家湾组紫红色砾岩的沉积时间在835～864Ma。板溪群马底驿组砂岩给出了764Ma和812Ma的年轻年龄峰值,结合其他资料推测其主体沉积时间应在760～720Ma之间。上述资料表明以往所认为的冷家溪群沉积于中元古代的观点需要重新拟定为新元古代,冷家溪群与银珠坝组或宝林冲组下部之间的角度不整合(835～864Ma),代表了扬子与华夏陆块沿江南隆起带碰撞的结束时间,而扬子周边广泛发育的750～830Ma的岩浆作用是晋宁造山之后的一次大规模板内岩浆作用。更多还原

【Abstract】 This paper presents the field investigation and zircon U-Pb geochronological data for the Neoproterozoic basal conglomerates and the associated rocks at the Cangshuipu area (Yiyang),Hunan Province. The results show that the Neoproterozoic sequences in this area are composed of the Lengjiaxi,Cangshuipu and Banxi Groups with the Hengluchong conglomerates at the basal part. The Cangshuipu Group consists of the Lingjiawan (the red basal conglomerate) and Yinzhuba (terrestrial volcanic rock) Formations. Geochronological studies of the detrital zircons from the sandstone samples from the upper part of the Lengjiaxi Group,which is overlain by the red basal conglomerates of the Linjiawan Formation,exhibit the youngest age-peak of 864Ma. The zircons separated from sandstone sample from the Madiyi Formation of the Banxi Group yield the youngest age-peak of 764Ma. The age-peaks of 864Ma and 764Ma limit the maximum depositional age of the Lengjiaxi and Banxi Groups,respectively. The SIMS zircon U-Pb dating of the volcanic sample from the Yinzhuba Formation that underlain the Linjiawan conglomerates yields a weighted mean 206Pb/238U age of (835±12)Ma,representing the eruption age of the Yinzhuba volcanic sequences. Integrating with the other geological data,it is inferred that the Lengjiaxi Group might be deposited in Neoproterozoic rather than the previously proposed Meoproterozoic.. The conglomerates of the Linjiawan Formation were most likely deposited in the interval of 864~835Ma. This age probably represents the final amalgamation between the Yangtze and Cathaysian Blocks along the Jiangnan Mountains.更多还原