Origin of chert in Lower Silurian Longmaxi Formation: Implications for tectonic evolution of Yangtze Block, South China

Abstract

Upper Ordovician (Katian) to Lower Silurian (Rhuddanian) strata in the Middle–Upper Yangtze Block, South China contain extensive chert beds, although the origin of microcrystalline silica in these cherts is largely unknown. In this study, the mineralogy and structure of chert as well as major, trace, and rare earth elements are used to determine the origin of microcrystalline silica in 29 chert samples from Longmaxi Formation (Rhuddanian) in Baizitian outcrop, west of Kangdian Uplift (KU). Results show that 21 of the 29 chert samples have a biogenic origin, 2 indicate a hydrothermal origin, and 6 show a mixed origin.Cherts from Baizitian outcrop are compared to contemporaneous cherts deposited east of KU. Cherts both east and west of KU are mainly biogenic in origin, which is attributed to high productivity in the paleo-seaway. Hydrothermal microcrystalline silica is relatively uncommon in the Longmaxi Formation across the Middle-Upper Yangtze Block, and only occurs in Baizitian outcrop. Terrigenous influx contributed to chert accumulation both east and west of KU; however, cherts east of KU received much more clastic material than those in Baizitian outcrop.A comparison of chert composition and geochemistry to geological reconstructions of the Middle-Upper Yangtze Block reveal that accumulation of hydrothermal chert is closely linked to tectonic evolution. During the Early Silurian, the Middle-Upper Yangtze Block was in a compressional tectonic regime with limited heat supply, and hence, low hydrothermal circulation; this is manifested in the paucity of hydrothermal microcrystalline silica in cherts east of KU. In contrast, hydrothermal microcrystalline silica in cherts west of KU is interpreted to have formed in an extensional tectonic regime with sufficient heat supply. This research demonstrates that chert source is, in part, tectonic controlled, and chert geochemistry can be used in reconstructing paleoenvironments.