Geochemistry of upper Permian siliceous rocks from the Lower Yangtze region, southeastern China: implications for the origin of chert and Permian ocean chemistry

• 来源期刊: 石油科学（英文版）
• 年，卷（期）: 2019,（2）
• 页码范围: 252-266
• 页数: 15页
• 语种: 英文

摘要

The Permian Chert Event is of great significance to understanding the geological evolution of the entire Permian;however,the origin of widespread chert formation is debated.We report new geochemical data from deep-marine siliceous rocks of the upper Permian Da-long Formation,Lower Yangtze region,southeastern China.Their geochemical results show that these thin-bedded siliceous rocks have a clear biologic origin,with rare to no evidence of hydrothermal influence.The values of Al/(Al + Fe + Mn) and Eu/Eu* are 0.60-0.84 (mean =0.72) and 0.45-1.08 (mean =0.77),respectively,and Mn/Ti ratios are relatively low (mean =0.72).The correlations of LaN/CeN,LaN/YbN,and Fe2O3/TiO2 with Al2O3/(Al2O3 + Fe2O3),along with the Ce anomaly,indicate that the Da-long siliceous rocks were deposited at a transitional zone between a continental margin and the open ocean;i.e.,relatively close to terrestrial sediment input and far from hydrothermal activity.The accumulation of chert is related to its unique paleogeographic location in an equatorial setting with many submarine paleo-highlands.Intense upwelling and frequent local volcanism are the main factors that promoted the development of siliceous rocks in the studied area.Ocean acidification triggered by large-scale volcanism (Large Igneous Province) during the late Permian led to extensive silica precipitation and preservation.