Elemental geochemistry of the early Jurassic black shales in the Qiangtang Basin, eastern Tethys: constraints for palaeoenvironment conditions

摘要

The early Jurassic black shales are the most important source rocks in the Qiangtang Basin, Tibet. These black shales may provide a useful example for understanding the sedimentary conditions during the early Jurassic in the eastern Tethys. Thirty-two black-shale samples were collected from the Qiangtang Basin to reconstruct the depositional environments during the early Jurassic in the eastern Tethys. The palaeoclimate index sigma(Fe+Mn+Cr+Ni+V+Co)/sigma(Ca+Mg+Sr+Ba+K+Na) varies between 0.58 and 1.14, together with low Sr/Ba (0.20-0.60) and Sr/Cu (1.68-7.88) ratios, indicating a warm humid to semi-humid climatic condition during black-shale deposition. The early Jurassic black shales exhibit slight Ce anomalies (0.89-0.94) and Mn enrichment (EF=1.2), suggesting a slightly oxic depositional environment. The conditions of the sedimentary water system during black-shale deposition were apparently brackish as evidenced by middle B/Ga (3.96-5.73) and Th/U (3.96-6.51) ratios. Copyright (c) 2015 John Wiley & Sons, Ltd.