Origin of Carnian Ma’antang cherts, northwestern Sichuan Basin, South China: Field, petrographic, and geochemical perspectives

Abstract

Abundant Carnian bedded and nodular cherts occur in the Sichuan foreland basin, South China. These cherts faithfully record the Carnian Pluvial Episode and the earliest evolution of the Sichuan foreland basin. However, importantly, the formation mechanisms of the cherts and the source of silica remain controversial. To understand the origin of these cherts, we performed a multitracer study by combining field logging, petrological analyses, Si and O isotope analysis, and major and trace element analysis of chert samples from two stratigraphically correlated sections. The Ma'antang cherts have low Ge/Si ratios (∼0.02 μmol/mol) and display flat seawater-like rare earth element plus Y (REY) distributions with (Pr/Yb)PAAS < 1 and superchondritic Y/Ho ratios (Y/Ho ≥ 27). In addition, siliceous sponge body fossils and spicules appear both in the field and in thin sections, implying a biogenic origin. Insignificant negative Ce anomalies (0.83–1.54; average 1.06) indicate REY contributions to anoxic ferruginous seawater. The heterogeneous distribution and high values of stable Si isotope compositions (δ30Si = 0.5‰-1.2‰) and the very low Ge/Si molar ratio for microcrystalline matrix quartz clearly reveal biogenic (e.g., sponge-derived) silica as the Si source, excluding hydrothermal silica. The chert precipitation occurred under the temperatures of ∼29 to 33 °C, as determined using an oxygen isotope thermometer. Quantitative simulation based on isotopic and REY data suggest that Ma’antang cherts were frequently affected by freshwater influx into marine water. We propose that the biogenic extraction and precipitation of silica from upwelling seawater was the source of silica for cherts in Upper Triassic oceans.