云南兰坪盆地云龙组硫酸盐硫同位素特征及其地质意义

Sulfur isotopic composition of sulfate and its geological significance of the Yunlong formation in the Lanping Basin,Yunnan Province

位于三江特提斯南段的兰坪盆地，发育云龙组含石膏‐石盐的蒸发岩系，这些蒸发岩系不仅对钾盐研究，而且对著名的金顶铅锌矿的成因有着重要意义。近年来，在其南端思茅盆地的研究提出了可以与云龙组相对比的勐野井组蒸发岩为新特提斯自西而东的海侵所致。那么，兰坪盆地沉积的蒸发岩是否也是来自于同时期的新特提斯海水？因此，通过系统的露头采样，本文开展了岩相学和石膏硫同位素研究。结果表明，兰坪金顶、小盐井、拉井和河西乡原生石膏样品δ34 S值在12．6‰～17．6‰之间，且主要集中在13‰～15‰范围内，几乎没有受到细菌还原作用的影响。这与思茅盆地勐野井组下段硬石膏、老挝他曲盆地含钾盐塔贡组基底硬石膏和泰国坷叻盆地含钾盐马哈萨拉堪组基底硬石膏的硫同位素值一致。云龙组石膏硫同位素值大部分样品落于白垩纪海水δ34 S范围内，表明石膏为海相成因。根据前人古地理和古地磁资料，推断在晚白垩世时期，由于新特提斯洋的海侵，在兰坪‐思茅‐万象‐坷叻形成了相互连通的蒸发盆地系。

The Lanping Basin is situated in the south part of the Sanjiang Tethys .The Yunlong Formation in the Lanping Basin contains gypsum‐rock salts ,which is significant to origin of potash salts and Jinding Zn‐Pb deposit .Recent advances in the Simao Basin inferred that evaporates in the equivalent Mengyejing Formation originated from marine incursion of the Neo‐Tethys Ocean .Did evaporates of the Yunlong Formation originate from the Neo‐Tethys Ocean . A combined analysis of petrology and sulfur isotopic geochemistry was performed .The results show that sulfur isotope values of primary gypsum samples from Jinding ,Xiaoyanjing ,Lajing ,and Hexi are between 12 .6‰ ~17 .6‰ ,with most values in the range of 13‰ ~15‰ .These values ,which have no effect of bacteria sulfate reduction ,are similar to that of contemporaneous marine anhydrites in the Vientiane and Khorat basins and anhydrite in the Mengyejing Formation .Most values are in the range of δ34 SV‐CDT of the Cretaceous sea water indicating marine origin . Together with previous palaeogeographic and palaeomagnetic data ,we speculated that a vast evaporatic basin＆amp;nbsp;system was formed by marine incursion during the Late Cretaceous .