东特提斯域白垩纪中期思茅-呵叻海及其与巨量钾盐形成的关系

The Khorat Basin contain immense potash deposits in southern part of eastern Tethyan realm, while Mengyejing potash deposit in the Simao Basin is the only pre-Quaternary potash salt deposit in China, with proved reserves of 20 million tons. The formation mechanism of these potash salts has always been the long standing arguments due to huge differences in potash resource including the metallogenic ages, material sources, and genetic links of brines. Our recent research shows that the ages of potash-bearing formations are the same in the Simao and Khorat basins. Several marine incursions supply the material to potash salts evaporation. Therefore, we propose a mid-Cretaceous Simao-Khorat Sea model, which is formed by marine incursion of the Meso-Tethyan Ocean, to explain the unsolved disputes. This research focus on the mid-Cretaceous evaporates-bearing of the Maha Sarakham Formation in the Khorat Basin. Organic geochemistry, isotopic geochemistry, and composition of halite-sylvite fluid incursions with sedimentology, paleontology analysis are performed to build the chain of evidences of marine incursion in the mid-Cretaceous and ascertain the periods and direction of the marine incursion in the Khorat Basin, aiming to reveal the relationship between the Simao-Khorat Sea and potash mineralization. The results are not only providing proof to “The distribution and regularity of important mineral resources in Tethyan Realm”, but also of importance to guide for further potash exploration in the Simao Basin and support the “Belt and Road ” strategy.

东特提斯域南端的呵叻盆地蕴藏有巨量钾盐沉积，而我国仅在思茅盆地勐野井发现有2000万吨储量，其资源量的巨大差异使得其成因机制是长期的争论焦点，涉及到成矿时代、成钾物质来源和成矿卤水的成因联系等方面。申请者近年来的研究表明，思茅与呵叻盆地含钾地层时代一致，思茅盆地成钾物质可能来源于海水且经历了多次海侵。由此，本项研究提出可能存在一个白垩纪中期由中特提斯洋海侵形成的思茅-呵叻海，这个模型可以合理解释目前存在的诸多争议。本项研究拟以呵叻盆地马哈萨拉堪组为研究对象，以有机和同位素地球化学及盐类矿物包裹体成分为主要手段，辅以沉积学、古生物学研究，开展思茅-呵叻海存在证据、沟通方式和通道的研究，查明思茅-呵叻海与巨量钾盐成矿的关系。研究成果不仅将为实现重大研究计划总体目标“特提斯域内重要矿产资源的分布与形成规律”提供了依据，而且对思茅盆地找钾具有重要实践意义，也可为“一带一路”国家战略提供科学支撑。

白垩纪时期，受全球海平面高企，印度板块持续北进，羌塘-拉萨地体完全碰撞导致海水彻底退出等事件的影响，叠加副热带高压，在思茅-呵叻盆地形成了巨型含钾盐蒸发岩盐系，然而盆地间钾盐资源却出现巨量差异。项目提出了思茅-呵叻陆表海的工作模型，试图回答思茅和呵叻盆地巨量钾盐差异沉积的机制。三年来，本项目聚焦呵叻盆地，开展了海侵证据链求证、白垩纪源-汇分析和古水系展布、古地理重建等工作，取得了如下成果。第一，建立多学科证据链，论证了思茅-呵叻白垩纪中期陆表海模型的成立，并据此初步解释了原-副特提斯海侵和巨量钾盐差异成矿的机制。首次在呵叻盆地含盐层系有机质中发现了特征性的海相生物标志物24-正/异丙基胆甾烷；发现石盐原生流体包裹体成分与白垩纪时期古海水一致；近期发表的石盐/石膏B和Sr同位素也反映了典型的海相特征，从而多角度地论证了海侵的存在。进一步通过古地理分析，认为是原-副特提斯海侵为成钾带来了成矿物质，海退时水气减少加剧了盆地的干旱。高分辨率Br含量剖面显示呵叻盆地成盐时期为浅水盆地，更从侧面证实了思茅-呵叻陆表海盆地的可靠性。第二，提出在122 Ma，盆地物源发生根本变化，从122 Ma之前以古湄公河跨大陆水系输送的松潘-甘孜、羌塘碎屑物源为主转变为之后以邻近的Sibumasu岩浆岩物源为主，物源区的隆起导致了盆地干旱化加剧，发育风成砂和蒸发岩。大河水系的瓦解可能反映了区域构造隆起的影响，使得水系发生重组，盆地也有外流向内流水系转变。这些发现证实思茅和呵叻盆地在122 Ma之后为独立的盆地，可能仅在海侵期相连。研究结果对揭示巨量钾盐差异沉积提供了独特视角，也可为思茅盆地的钾盐资源前景提供依据。