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.