Mixed seawater and hydrothermal sources of nodular chert in Middle Permian limestone on the eastern Paleo-Tethys margin (South China)

Abstract

Middle Permian limestone and marl of the Maokou Formation deposited along the eastern Paleo-Tethys margin (South China) host abundant chert nodules that formed in association with the global Permian Chert Event (PCE). An enhanced understanding of the origin of the chert may help elucidate Middle Permian oceanic and diagenetic conditions. However, the mechanisms responsible for the formation of chert, especially the source of silica, remain controversial. Maokou chert nodules, which are made up of quartz and subordinate calcite, hematite, pyrite, and fluorite, and rimmed by F-bearing sepiolite laminae, appear to have formed mainly by the replacement of precursor nodular limestone by silica. The nodular limestone may have formed by differential compaction of interbedded limestone and marl, after which carbonate was replaced gradually outward from nodule centers by dissolved silica released from the dissolution of siliceous skeletal grains (including spicules and radiolarians) in host marl during early diagenesis. The introduction of high-T hydrothermal fluids into ocean water is evidenced by the presence of F-bearing minerals (fluorite and sepiolite), a hybrid rare earth element (REE) + Y pattern, and a wide range of δ30Si values (−0.4‰ to +0.7‰) documented from the Maokou nodular chert. Conservative estimation indicated that Middle Permian seawater may have had a δ30Si composition (+0.7‰ to +1.8‰) similar to that of modern open ocean water, suggesting that seawater was the dominant silica source of nodular chert. Mixing calculations based on REE + Y geochemistry suggest additional contributions of ca. 0.05–0.2% high-T hydrothermal fluid and ca. 1–10% of continental-derived freshwater to ocean water. Such increased influx of silica to seawater may have been related to the early rifting induced by Emeishan volcanism and intensified chemical weathering during an interglacial period, which played an important role in promoting the flourishing of siliceous organisms and subsequent chert enrichment during the Middle Permian.