Early Carboniferous black mudstones in the Nujiang Suture Zone in northeast Tibet: implication on paleoenvironment

Abstract

Early Carboniferous sedimentary rocks were distributed in various landmasses or microlandmasses and suture zones in the northeastern Tibet. The section of black mudstones of the lower Carboniferous Bangda Formation was found at Kaqiong microlandmasses of Nujiang Suture Zone north of Lhorong County. The section is composed of black silty mudstone and mudstone subjected to slight metamorphism. In order to determine the paleoenvironment, provenance, and tectonic background of black mudstones, geological survey and geochemical data of this section were discussed. The results show that the TOC content of black mudstones ranges from 1.86 to 13.14%, and the organic matter is mainly sapropelic. The black mudstones with lower Sr/Ba and Sr/Cu values indicate a warm and humid climatic environment during the sedimentary period. The higher U (authigenic), U/Th, and Ni/Co values of mudstones indicate the reduced environment in the sedimentary period, but there is a rapid oxidation environment in the middle of the sedimentary period. This abrupt change results in the “two-stage” distribution of TOC and related geochemical properties. Total REE concentrations of mudstones range from 155.9 to 346.2 μg/g. All samples have chondrite-normalized REE patterns with enrichment of LREE, depletion of HREE, and negative Eu anomaly. The binary diagrams of Ni-TiO2 and La/Th-Hf and the Al2O3/TiO2 ratios indicate that the black mudstone mainly derived from felsic provenance. The discrimination diagrams revealed that the source rocks were mostly deposited in a collision setting.