扬子西缘新元古代中期裂谷作用:来自年代学与沉积学的新证据

New Sedimentological and Geochronological Evidence for Mid-Neoproterozoic Rifting in Western Yangtze Block,South China

华南新元古代沉积盆地演化与Rodinia超大陆裂解存在紧密联系,但仍缺乏精细刻画.对扬子西缘澄江组开展了系统的碎屑锆石年代学和沉积学研究.澄江组砂岩的碎屑锆石U-Pb年龄主要为新元古代(870~780 Ma),少数为前新元古代(2850~1010 Ma),最显著的峰值为820 Ma,最年轻一组碎屑锆石206Pb/238U年龄加权平均值为804.5±5.4 Ma.结合已发表年龄数据,将澄江组沉积时限进一步限定为800~720 Ma.物源分析揭示澄江组的新元古代碎屑锆石剥蚀自邻近的新元古代岩浆岩,而前新元古代锆石可能来自于邻近新元古代岩体的剥蚀或地层的沉积再循环.扬子西缘新元古代中期沉积盆地具有由冲积扇相逐渐过渡为前扇三角洲相的沉积演化序列,最终形成了具有裂谷充填特征的"楔状地层".这种沉积超覆演化过程在整个华南新元古代裂谷系普遍存在,指示在800 Ma左右华南全面进入裂谷盆地成熟阶段.

Although the evolution of the mid-Neoproterozoic basins in South China is closely related to the Rodinia supercontinent breakup,it still lacks fine characterization.In this study,detrital zircon LAICP-MS U-Pb dating and systematic sedimentary analyses were performed on the Chengjiang Formation in the western Yangtze Block.Most of the clastic zircon U-Pb ages in Chengjiang Formation are Neoproterozoic(870-780 Ma),and a few are pre-Neoproterozoic(2850-1010 Ma).The most significant peak of U-Pb ages in Chengjiang Formation is 820 Ma,and the youngest age population yields a weighted mean of804.5±5.4 Ma.Taking into account the available age data,it is suggested that the deposition time of Chengjiang Formation is further limited to 800-720 Ma.The Neoproterozoic detrital zircons of Chengjiang Formation were mainly denuded from neighboring Neoproterozoic magmatic rocks.The pre-Neoproterozoic zircons may come from the denudation of preNeoproterozoic pluton or the sedimentary recycling of strata.Mid-Neoproterozoic sedimentary rocks on the western margin of the Yangtze have a sedimentary evolution sequence that is characterized by gradual transitions from alluvial fan facies to anterior fan delta facies,and finally forms a"wedge formation"with rift filling characteristics.The sedimentary overlap process is common throughout the South China rift system,indicating that South China entered the maturity stage of the rift basin at 800 Ma.