渝东南新元古界千子门组沉积时代和构造背景——锆石UPb年龄及地球化学制约

新元古代全球范围内出现了2次主要的冰期事件, 分别是Sturtian冰期和Marinoan冰期。渝东南秀山地区发育与上述2次冰期对应的冰川沉积, 分别为南沱组(Marinoan冰期)和千子门组(Sturtian冰期)。笔者通过对千子门组砂岩锆石运用LA-ICP-MS进行定年, 测得其206Pb/238U年龄为736～2 726 Ma, 其中28个测点数据非常集中, 在736～890 Ma, 平均值为834 Ma。其最小值736 Ma限定了Sturtian冰期千子门组的沉积起始时间, 且与湘东南江口冰期起始时间(734±4) Ma在误差范围内几乎一致, 二者可以对比。而834 Ma则记录了物源区与四堡—晋宁运动有关的岩浆活动。所有样品ICV值介于0.85～1.74(平均值为1.23), 表明黏土矿物较少, 受再沉积作用的影响较小。同时其CIA值介于43～69(平均值为57), 表明千子门组沉积环境特征为寒冷、干燥, 低程度化学风化。利用SiO2-K2O/Na2O和K2O/Na2O-SiO2/Al2O3构造环境判别图解, 结合稀土元素配分曲线, 判定千子门组很可能形成于活动大陆边缘环境。

Depositional Age and Tectonic Settings of Neoproterozoic Qianzimen Formation in Southeast Chongqing:U-Pb Dating and Geochemistry Constraint

There are two major glacial events in Neoproterozoic globally, respectively the Sturtian glacial stage and Marinoan glacial stage.Corresponding glacialde posits develop in Xiushan area of Southeast Chongqing, respectively Nantuo formation and Qianzimen formation. ZirconLA-ICP-MS dating of Qianzimen sandstone obtains 206Pb/238 U age of 736—2 726 Ma, in which the data of 28 measuring points concentrate between 736 and 890 Ma, with anaver age of 834 Ma. Minimum value of 736 Ma defines the sedimentary starting time of Qianzimen formation, which can be compared with Jiangkou glacial stage of southeastern Hunan started in 734±4 Ma, almost unanimously in the same error range.The age of 834 Ma records magmatism related to Sipu-Jinning orogeny movement in provenance.ICV numerical values of all samples being 0.85—1.74(1.23 on average) show that clay minerals are less and Qianzimen formation is less affected by the redeposition. Meanwhile, CIA numerical values of the samples being 43—69(57 on average)indicatethe sedimentation environment of Qianzimen formation is cold, dry and low degree of chemical weathering.SiO2-K2O/Na2O and K2O/Na2O-SiO2/Al2O3 discrimination diagrams and REE patternscurve indicate that Qianzimen formation is created in the active continental margin environment.