华南扬子陆块成冰纪冰川作用的启动时限及其全球对比*

江口冰期和南沱冰期是华南地区引人注目的2次成冰纪冰川事件,但其确切启动时间及其全球对比关系仍未有定论。为此,对桂北地区成冰系(南华系)长安组底部和南沱组底部冰成杂砾岩开展了碎屑锆石U-Pb年代学研究。长安组碎屑锆石U-Pb年龄集中分布于958—717 Ma,显著峰值为720、753、805及848 Ma,最年轻一组 ²⁰⁶Pb/²³⁸U 年龄的加权平均值为719.6±6.1 Ma,可解释为长安组最大沉积年龄;南沱组碎屑锆石U-Pb 年龄集中分布于987—649 Ma,显著峰值为650、720、753、779、803、823及848 Ma,最年轻一组 ²⁰⁶Pb/²³⁸U 年龄的加权平均值为649.3±6.2 Ma,可解释为南沱组最大沉积年龄。结合已发表的相关年龄数据可知,江口冰期很可能启动于ca.715 Ma,与塔里木、阿拉伯—努比亚、劳伦等陆块的Sturtian冰川作用高度同步;南沱冰期的启动应晚于650 Ma,与西伯利亚、澳大利亚、劳伦等陆块的Marinoan冰川作用基本同步。另外,碎屑锆石U-Pb年龄谱与CL图像显示,长安组和南沱组的物质来源主要为下伏新元古界岩浆—沉积记录,揭示出冰川对下伏地层的强烈刨蚀作用和华南新元古代幕式构造岩浆热事件。扬子陆块成冰纪冰川刨蚀作用可能与Rodinia 超大陆“裂离”有关的强烈伸展活动存在联系,并可能持续至Marinoan 冰期结束。

Onset time and global correlation of the Cryogenian glaciations in Yangtze Block,South China

It's well known that the Jiangkou and Nantuo glaciations are two remarkable Cryogenian glaciations in South China. However,their precise onset time and global correlations remain undecided. In this study,detrital zircon LA￣ICP￣MS U-Pb dating was performed on the glacial diamictites from the basal part of the Chang'an and Nantuo Formations in the southeastern Yangtze Block. The new results show that detrital zircon grains from the Chang'an Formation give a concentrated ²⁰⁶Pb/²³⁸U age range of 958 to 717 Ma with distinct peaks at 720,753,805 and 848 Ma. The youngest age population yields a weighted mean of 719.6±6.1 Ma,representing the maximum depositional age of the Chang'an Formation. Detrital zircon grains from the Nantuo Formation has a main ²⁰⁶Pb/²³⁸U age range between 987 and 649 Ma,with distinct peaks at 650,720,753,803,823 and 848 Ma. The youngest age population gives a weighted mean of 649.3±6.2 Ma,representing the maximum depositional age of the Nantuo Formation. Taking into account the available age data,it is suggested that the Jiangkou glaciation was most likely initiated at ca.715 Ma,which is synchronous with the Sturtian glaciation in other blocks such as Tarim,Arabia-Nubia and Laurentia;the Nantuo glaciation was probably initiated later than 650 Ma,synchronous with the Marinoan glaciation in other blocks such as Siberia, Australia and Laurentia. Furthermore,detrital zircon U-Pb age spectrums and CL images indicate that sediments of the Chang'an and Nantuo Formations in the Yangtze Block were mainly derived from the underlying Neoproterozoic magmatic and sedimentary rocks,indicative of the intensive Neoproterozoic severe exaration and episodic tectonic-thermal events. The Cryogenian exaration in the Yangtze Block should be the response to the breakup of the Rodinia supercontinent prior to the termination of the global Marinoan glaciation.