川西列古六组底部凝灰质杂砾岩锆石U-Pb年龄及其意义

ZIRCON U-Pb AGE DATING ON THE TUFACEOUS DIAMICTITE FROM THE BOTTOM OF THE LIEGULIU FORMATION IN THE WEST SICHUAN AND ITS SIGNIFICANCE

长期以来,川西地区列古六组一直被认为与新元古代冰川晚冰期(Marinoan)地层南沱组相当,从而认为扬子西缘缺失早冰期(Sturtian)沉积,但缺少可靠的年代学证据。针对川西列古六组底部凝灰质杂砾岩锆石LA-ICP-MS U-Pb定年结果表明,^(206)Pb/^(238)U年龄范围为722 Ma^902 Ma,在~725 Ma,~780 Ma,~820 Ma和~850 Ma具有明显的峰值。其中,最年轻一组^(206)Pb/^(238)U年龄加权平均值为724 Ma±12 Ma,代表列古六组底部杂砾岩的最大沉积时间。列古六组沉积序列特征以及华南Sturtian冰期启动年龄分布范围综合分析可知,川西列古六组下段为早冰期沉积,可与长安组对比,而上段则为间冰期沉积,可与富禄组和大塘坡组对比。因此,扬子西缘存在早冰期(Sturtian)和间冰期沉积,不存在晚冰期(Marinoan)沉积。

The Lieguliu Formation in western Sichuan has always been correlated to the Nantuo Formation of the late glaciation （Marinoan） and it has tong believed that there were no early glaciation （Sturtian） deposits in the western Yangtze Block, though,it lacked valid chronology evidence. LA-ICP-MS U-Pb dating for the tuffaceous diamictite at the bottom of the Neoproterozoic Lieguliu Formation in western Sichuan showed that the ^206Pb/^238U ages range from 722 Ma to 902 Ma with distinct peaks at ～725 Ma,～780 Ma,～820 Ma ands850 Ma. Among them,the weighted average ^206Pb/^238U age of the youngest zircons is 724 Ma±12 Ma,which represent the maximum deposition age of the tuffaceous diamictite at the bottom of the Lieguliu Formation. Based on sedimentary successions of the Lieguliu Formation and the onset ages of the Sturtian glaciation in south China,it is inferred that the lower part of the Lieguliu Formation belongs to the early glaciation deposit and should be correlated to the early glacial stratum,the Chang＇an Formation; and the upper part of the Formation may be correlated to the interglacial strata, Fulu Formation and Datangpo Formation. Therefore,it is concluded that only early glaciation （Sturtian） and interglacial deposits occurred and there was no late glaciation （Marinoan） deposits in the area.