Tectonic control of Guadalupian-Lopingian cherts in northwestern Sichuan Basin, South China

Abstract

Abstact Cherts have been considered to be formed mainly through processes involving biosilicification, riverine inputs and hydrothermal activities. In the Permian, massive cherts are widely distributed in South China, bearing significant implications for the reconstruction of paleogeography and paleoceanography of the eastern Paleo-Tethys Ocean. The origin of the Permian cherts in South China, however, remains inconclusive due to a lack of systematic petrographic and geochemical study. In this article, we analyze the petrologic and geochemical characteristics of the cherts collected from five outcrops in the northwestern Sichuan Basin, South China. These cherts are found from the Upper Guadalupian Maokou Formation (ca. 268–259 Ma) and Upper Lopingian Dalong Formation (ca. 254–252 Ma). Field and microscopic observations suggest that cherts from the Maokou Formation occur mainly in nodular forms, and consist of microcrystalline quartz, silicified bioclasts and siliceous fossil debris. Both petrographic observation and geochemical analysis suggest that the Maokou cherts are mainly of hydrothermal origin near deep faults in the southwest and of biogenetic origin with less hydrothermal influence in the northeast of the studied area. The Dalong cherts occur in both nodular and bedded forms, with more abundant siliceous fossil debris than the Maokou cherts. Major, trace and rare earth element data support that the Dalong cherts are influenced by both biosilicification and hydrothermal activities. The spatial heterogeneity of the cherts origin in South China is largely controlled by tectonic activities associated with the Emeishan large igneous province (ELIP) event and the closure of the Paleo-Tethys Ocean. The troughs formed by the extensional tectonic activity linked to the ELIP could provide accommodation space and activate deep faults allowing the rise of silicon-rich hydrothermal fluids from the deep and nutrient-rich water from volcanogenic upwellings. Subsequent cooling may have facilitated the precipitation of large amounts of silica and the formation of cherts in the study region. This work demonstrates that the origin of the Guadalupian-Lopingian cherts in South China is predominantly controlled by tectonic and biological activities, which sheds light on the global “Permian Chert Event”.