REE geochemistry and sedimentary-tectonic setting of the Early Carboniferous black rock series in southern Guizhou

In order to explore the sedimentary-tectonic setting of the Early Carboniferous black rock series in southern Guizhou, the emphasis of the present paper is placed upon the REE geochemistry of the Early Carboniferous black shale samples from the CY1-well through the Dawuba Formation in the Changshun region, southern Guizhou. The results of research show that the ΣREE values for the Early Carboniferous black shales vary from 213.08×10^-6 and 308.1×10^-6, with an average of 251.43×10^-6 greater than that(173.21×10^-6) for the North American Shale Composites(NASC). The geochemical parameters such as w(ΣLREE)/w(ΣHREE) and w (Ce)_N/w(Yb)_N ratios indicate the highly differentiation of ΣLREE and ΣHREE. The smooth REE distribution patterns display the enrichment of LREE and the depletion of HREE. The δEu values range from 0.53 to 0.67(<1), indicating the moderately negative Eu anomalies and slightly Ce anomalies. The changes in δCe and Ce anomalies suggest a redox environment of the study area during the Early Carboniferous. It is inferred from the REE distribution patterns and relevant parameters such as δEu values and w(La)_N/w(Yb)_N vs. ΣREE diagram in combination with regional geological data that the principal source of the black shales from the Dawuba Formation in southern Guizhou lay to the Central Guizhou uplift and Xuefengshan uplift, and the source rocks are interpreted as the mixtures of granites and sedimentary rocks. The tectonic setting of the study area should be attributed to the passive continental margin.