羌塘盆地双湖地区曲色组冷泉碳酸盐岩及其地质意义

羌塘盆地南部双湖地区曲色组地层发育大量碳酸盐岩结核,这些结核多呈丘状、椭球状、透镜状、似层状、脉状、树枝状产出。其物质组分主要为泥微晶碳酸盐矿物,少量粘土矿物、石英及草莓状、半自形黄铁矿等。草莓状黄铁矿平均粒径在5.0μm左右,内部可见葵花状构造。发育凝块状、气孔状和渗漏孔等特殊构造。产双壳类、菊石类、蠕虫状或树枝状生物及超微生物化石,生物密度极高。碳同位素明显负偏,硫同位素则明显正偏,其特征与现代海底天然气水合物的渗漏、释放所形成的冷泉碳酸盐岩机理一致,因而推测为古代海底天然气渗漏喷发形成。大量海底天然气泉口的存在,可能表明羌塘盆地双湖地区早侏罗世大洋缺氧事件与海底天然气水合物喷发存在极大关联。

Characteristics and significance of the cold-vent carbonate rocks from the Quse Formation in the Shuanghu area, Qiangtang Basin, northern Xizang

The carbonate concretions occur in large numbers in the Quse Formation, Shuanghu area, Qiangtang Basin, northern Xizang, and display hummocky, ellipsoidal, lenticular, stratoid, vein, dendritic, clotted, vesicular and seepage textures and structures. These nodules consist dominantly of micritic carbonate minerals with minor amount of clay minerals, quartz, framboidal and subhedral pyrite with an average grain size of about 5.0 μm,in which the sun-flower structures may be observed. The organic fossils such as bivalves, ammonites, worm-like or dendritic fossils and ultra-microbial fossils are found to be arranged with a higher density. The carbon isotopes in the analyzed samples are noticeably negative excursion while the sulfur isotopes in the analyzed samples exhibit a markedly distinct positive excursion. The results of research outlined above are in good agreement with the mechanisms for the formation of the cold-vent carbonate rocks produced by the seepage and release of the modern submarine natural gas. It is inferred from this reason that the carbonate concretions in the study area may be originated from the seepage and eruption of ancient natural gas. The presence of numerous submarine natural gas vents implies that there is a direct correlation between the oceanic anoxic events and the eruption of the submarine natural gas hydrates in the Shuanghu area, Qiangtang Basin during the Early Jurassic.