西藏地区晚白垩世日喀则弧前盆地冷泉碳酸盐岩的发现及其地质意义

Hydrocarbon seeps, widely occurring in present continental margins, are get increasingly focused owing to their close relationship with gas hydrate, strong greenhouse methane gas, and biological resources in extreme environment. Seep carbonates are byproducts of anaerobic oxidation of the emanated methane, which could be used to indicate past seepage activities and sedimentary characteristics. Ancient hydrocarbon seeps are increasingly recognized worldwide, but they are rarely found from ancient strata in the mainland China. Only two possible seep carbonates are documented: a controversial cap carbonates from the bottom of Doushantuo Formation in the Three Gorges area and the tentatively identified seep carbonates enclosed by late Cretaceous turbidites in the Xigaze forearc basin which was found by our research group. This project intends to study the sedimentology and the geochemistry of seep carbonates from the Xigaze forearc basin, Tibet, to address the gas seeping fluid origin and the sedimentary environment. More field work is designed to trace more hydrocarbon seep sites in the Xigaze forearc basin, and to develop a more comprehensive understanding of the time, locations, and scales of gas seepage. This project also aims to investigate the relationship between hydrocarbon seeps and gas hydrate dissociation, and the potential effect of methane seeps on the Cretaceous global climate and geological events, such as oceanic anoxic events.

冷泉广泛发育于大陆边缘海底，与天然气水合物资源、甲烷温室气体的环境效应和极端环境的生物资源等重大问题密切相关，因此受到越来越多的关注。冷泉碳酸盐岩是冷泉渗漏甲烷经微生物厌氧氧化形成的产物，可用来示踪冷泉流体活动和沉积环境特征。古代冷泉常以自生碳酸盐岩的形式保存在地层中，在我国大陆境内除有争议的三峡陡山沱组底部的盖帽碳酸盐岩和我们初步确定的日喀则晚白垩世碳酸盐岩结核外，尚未见报导。本项目拟对我们发现的西藏日喀则晚白垩世弧前盆地浊积地层中的碳酸盐岩结核进行详细的沉积岩石学和地球化学研究，确定冷泉活动的特征和流体的来源；开展空间上的野外地质工作，追踪相似的冷泉活动遗迹，认识日喀则弧前盆地烃类渗漏的时间、规模等分布特征，探讨渗漏流体与水合物分解的关系，评价甲烷渗漏事件对白垩纪气候变化和大洋缺氧等重大地质事件的影响。

项目对西藏日喀则晚白垩世弧前盆地浊积地层中的碳酸盐岩结核进行了详细的沉积岩石学和地球化学研究。在日喀则西南的卡堆村318国道北侧公路旁晚白垩世Turonian期昂仁组泥岩浊积层中发现了4层碳酸盐岩结核，结核主要由碳酸盐矿物（平均56.2%）、碎屑矿物（平均27.3%）及粘土矿物（平均16.5%）组成，碳酸盐矿物的δ13C值-27.7‰ ~ -4.0‰（V-PDB），为热解成因甲烷源，冷泉碳酸盐岩的Ce/Ce*值经La异常校正后显示无真Ce异常，形成于以弱还原为主的环境。该成果在Chinese Science Bulletin发表，是我国大陆第一个有确切证据的冷泉碳酸盐岩报导。. 在日喀则市卡堆村西南晚白垩世日喀则弧前盆地Cenomanian期深水浊积粉砂岩层中，发现有自生碳酸盐岩结核产出，重结晶作用和后期脉广泛发育，碳酸盐岩δ13C 值-34.1‰ ~ -12.9‰ (V-PDB)，δ18O值为-11.8‰ ~ -2.2‰ (V-PDB)。δ13C - δ18O和87Sr/86Sr - δ18O有很好的相关性，表明成岩过程中氧同位素与碳和锶同位素同时发生改造，依建立的碳酸盐岩原始同位素组成定量恢复的方法，计算的原始冷泉流体的δ13C值-34‰（V-PDB），成岩流体的87Sr/86Sr值为0.7072。该项成果于2016年在Chemical Geology 发表。