Petrographic, geochemical characteristics and genesis of the siliceous rock in the Middle Permian Maokou Formation at the eastern Sichuan Basin

ObjectiveTo explore the sedimentary environment, sources, and formation mechanisms of siliceous rock from the Maokou Formation on the eastern edge of the Upper Yangtze Block. MethodsThis study focuses on the siliceous rock in the Middle Permian Maokou Formation from the eastern Sichuan area. Based on detailed petrological observations and analyses, the major elements, trace elements, and rare earth elements of the siliceous rock were analyzed. ResultsThe results show that the formation of the siliceous rock was not influenced by terrigenous materials. The SiO2 primarily originated from hydrothermal fluids associated with Permian magmatism, with biological processes, which played a role in its formation. The siliceous rock formed in relatively hypoxic, deep-water rift troughs and surrounding shallow water platforms near the continental margin, under a background of alternating platforms and basins. The activation of deep basement faults caused by the Emei mantle plume and newly formed synsedimentary faults provided the main channels for the upward migration of hydrothermal fluids. The interaction between the hydrothermal fluids and the crust during migration resulted in characteristics indicative of crustal-mantle mixed sources. The siliceous rock was deposited during the synsedimentary period or shortly after deposition, mainly through the replacement of carbonate components. The loose and porous medium conditions facilitated the flow of hydrothermal fluids through the sediments, leading to bedding metasomatism or precipitation. The intermittent action of hydrothermal fluids caused uneven silicification. ConclusionThis research proposes a new perspective on the genesis of the Maokou Formation siliceous rock in the eastern Sichuan area of the Upper Yangtze Block, deepening the understanding of the paleotectonic sedimentary evolution at the Middle-Late Permian boundary, and providing important insights for natural gas exploration and development in this region.