青海南部沱沱河冻土水合物潜在区火山岩地球化学特征及地质意义

沱沱河盆地是冻土天然气水合物潜在分布区之一,其内发育下—中二叠统开心岭群九十道班组、上二叠统乌丽群那益雄组、上三叠统结扎群巴贡组、中—渐新统雅西措组4套烃源岩以及不同类型的火山岩。研究表明,火山岩岩石类型主要为玄武岩、玄武安山岩、安山岩、玄武质粗面安山岩和粗面安山岩。火山岩主量元素低TiO_2,Al_2O_3含量较高,K_2O含量较低,K_2ONa_2O;火山岩的稀土元素配分模式为轻稀土富集型;微量元素配分模式呈锯齿状,Ta、Nb、P_2O_5、TiO_2、Y、Yb以及铁族元素Sc、Cr、Ni亏损,综合判断青海南部沱沱河地区火山岩形成于岛弧环境。结合火山岩的地球化学特征,推断火山活动可以加快烃源岩的热成熟,进而促使有机质裂解,产生热解气,为水合物的生成提供气源条件。同时,火山岩的气孔构造发育,连通性较好,有可能成为水合物的储层。火山热液具有的高压使周围的烃源岩产生裂隙,这些裂隙不仅是水合物气源的运移通道,还可以为水合物提供形成场所和储存空间。

Geochemistry and geological implications of the volcanic rocks in the potential areas of the permafrost hydrates from the Tuotuohe area, southern Qinghai

The Tuotuohe area in southern Qinghai is one of the potential areas of the permafrost hydrates, where the source rocks are developed in the Lower-Middle Permian Jiushidaoban Formation of the Kaixingling Group, Upper Permian Nayixiong Formation of the Wuli Group, Upper Triassic Bagong Formation of the Jiezha Group, and Miocene-Oligocene Yaxicuo Formation. The volcanic rocks in the Tuotuohe area consist dominantly of basalt, basalitic andesite, andesite, basalitic trachyandesite and trachyandesite. These volcanic rocks are characterized by low TiO₂ contents, high Al₂O₃ contents, low K₂O contents and K₂O < Na₂O, and display the LREE-enriched REE distribution patterns, and zigzag trace element distribution patterns with the depletion of the elements Ta, Nb, P₂O₅, TiO₂, Y, Yb, Sc, Cr and Ni. Petrochemically, the volcanic rock samples are all projected into the basalt and andesite fields within the island-arc orogenic zone, as indicated by the ATK diagram. It is inferred that the volcanic rocks in the Tuotuohe area may be originated from the island-arc environments. The volcanic activities may facilitate the thermal maturity of source rocks, thermolysis of organic matter, generation of thermolysis gas and formation of hydrates. Meanwhile, the volcanic rocks may also interpreted as the reservoir rocks of the hydrates due to well-developed vesicular structures and connectivity. The fissures or cracks in the source rocks caused by volcanogenic hydrothermal solution may contribute to the pathways and spaces for the migration and preservation of the hydrates.