Geochemical features of the black shales from the Wuyu Basin, southern Tibet: implications for palaeoenvironment and palaeoclimate

The Mangxiang Formation black shales are the most important hydrocarbon source rocks in the Wuyu Basin. Trace and rare earth element (REEs) of the black shales from the Wuyu Basin were studied in order to understand their depositional environments and palaeoclimate. Thirty one black shale samples from the Wuyu Basin were analysed by inductively coupled plasma-mass spectrometry and X-ray fluorescence. The black shales are characterized by moderate SiO2 (52.55-58.73%) contents and medium K2O/Na2O (2.21-4.83) ratio values but relatively high MgO+Fe2O3 (5.8-6.91%) and Al2O3 (15.1-17.5%) contents. The Chemical Weathering Index of Alternation (CIA) ranges from 57 to 70, together with medium Th/U (2.66-4.97) ratio values, reflecting a weak to moderate degree of chemical weathering of the source area. The palaeoredox condition of black shale was slightly oxic (or dysoxic) during black shale deposition as evidenced by slightly Ce anomalies (0.92-0.99) and Mn enrichment (EF = 1.6). The moderate palaeosalinity values (9.32 parts per thousand-17.75 parts per thousand), together with medium B/Ga (3.90-5.57) ratio values, indicate a brackish water environment. The palaeoclimate index Sigma(Fe + Mn + Cr + Ni + V+ Co)/Sigma(Ca + Mg + Sr + Ba + K+ Na) ranges from 0.28 to 0.50 and low Sr/Ba (0.15-1.22) ratio values, indicating a semiarid to semimoist climatic condition during the sedimentation of the black shale. The high omega(La)(N)/omega(Yb)(N) ratio values (1.02-1.09) indicate a fast sedimentary rate during black shale deposition. In this study, a preservation model of the Mangxiang Formation black shale was established. The model indicates that excellent preservation may be the major controlling factor for the accumulation of organic matter. Copyright (C) 2015 John Wiley & Sons, Ltd.