塔里木地块西北缘阿克苏地区新元古代冰碛岩年代与冰期事件

Geochronology and Glaciation of the Neoproterozoic Diamictite in Aksu Area,Northwestern Margin of the Tarim Block

塔里木地块新元古代冰期事件等时性对比存在争议,获得可靠的新元古代冰碛岩沉积时代意义重大。塔里木地块西北缘阿克苏地区出露2套新元古代冰碛岩沉积,通过对其进行岩石学、同位素年代学、岩石地球化学等分析,明确其沉积物岩石组合特征、限定其沉积时代、讨论其古气候风化条件等。化学蚀变指数(CIA)指示研究区的2套新元古代冰碛岩代表了2次寒冷的冰川气候记录,分别为巧恩布拉克冰期和尤尔美那克冰期,其中巧恩布拉克冰期中出现2次次级冰期旋回。冰川沉积及其相邻层位的碎屑锆石U-Pb年代学数据显示,塔里木地块西北缘阿克苏地区巧恩布拉克冰期的起始年龄晚于(719±10)Ma;尤尔美那克冰期的起始年龄不会早于(685±11)Ma,其结束年龄可由南华系顶界年龄635 Ma或上覆震旦系苏盖特布拉克组内玄武岩层年龄(615±5)Ma限定。已有年代学、地球化学、沉积学等证据表明:巧恩布拉克冰期(阿克苏地区)与阿勒通沟冰期(库鲁克塔格地区)、江口冰期(华南地区)和Sturtian冰期(国际)相对应,尤尔美那克冰期(阿克苏地区)与特瑞爱肯冰期(库鲁克塔格地区)、南沱冰期(华南地区)和Marinoan冰期(国际)相对应。冰期事件、同位素年代学及地球化学数据可为塔里木地块新元古界的划分与对比提供依据。

Age correlation of the Neoproterozoic glaciation events in the Tarim Block remains controversial,and obtaining reliable deposition time for the Neoproterozoic diamictite is crucial.Two sets of the Neoproterozoic diamictite were developed in Aksu area along the northwestern margin of the Tarim Block.Petrologic and whole-rock geochemical analyses and radiometric dating were conducted to define the sedimentary rock assemblages,deposition time,and paleoclimatic weathering conditions.The chemical index of alteration(CIA)indicates that the Qiaoenbrak and Yuermeinak Formations represent two distinct glaciation episodes,and two secondary glaciation events exist in the Qiaoenbrak glaciation.Detrital zircon U-Pb dating of the glacial deposits and their adjacent beds indicate that the Qiaoenbrak glaciation in Aksu started along the northwestern margin of the TarimBlock after(719±10)Ma,whilst the Yuermeinak glaciation did not start before(685±11)Ma,and its cessation timing can be constrained by the top boundary age of the Nanhua System(635 Ma)or the U-Pb age of the overlying Sinian Sugetbrak Formation basalt(615±5 Ma).Comprehensive analyses of isotope geochronology,geochemistry,and sedimentology indicate that the Qiaoenbrak glaciation(Aksu area)corresponded to the Altungol glaciation(Quruqtagh area),the Jiangkou glaciation(South China),and the Sturtian glaciation(global).Meanwhile,the Yuermeinak glaciation(Aksu area)corresponded to the Tereekan glaciation(Quruqtagh area),the Nantuo glaciation(South China),and the Marinoan glaciation(global).Glaciation,isotopic geochronology,and geochemistry can provide a basis for the division and comparison of the Neoproterozoic sequences in the Tarim Block.