Petrography and geochemistry of Oligocene to Lower Miocene sandstones in the Baiyun Sag, Pearl River Mouth Basin, South China Sea: Provenance, source area weathering, and tectonic setting

摘要

There is a controversy regarding the source area and weathering intensity of Oligocene to Lower Miocene sediments in the Baiyun Sag of the Pearl River Mouth Basin. Detrital modes, geochemistry, heavy mineral data, and detrital zircon U-Pb ages were analysed to understand the provenance, source area weathering, and tectonic setting of Oligocene to Lower Miocene sandstones in the Baiyun Sag. Petrographic data reveal that the sandstones were derived from a recycled orogen. With increasing transport distance, sediment maturity increased correspondingly. The Oligocene to Lower Miocene sandstones in the Baiyun Sag have low compositional maturity and have experienced low to moderate weathering. The sediments of the Zhuhai Formation have experienced relatively strong weathering intensity compared with those of the Enping and lower Zhujiang formations. Due to low degree of illitization and differences in the geochemical compositions of the sediments, the continental island arc data from different boreholes in the Baiyun Sag are more irregularly distributed in the A-CN-K plot. The modal composition (e.g., quartz, feldspar, and lithic fragments) and geochemical indices (Al2O3/TiO2 ratio; Al2O3-TiO2 diagram; discriminant F1-F2 diagram; Eu/Eu*, La/Sc, La/Co, Th/Sc, and Th/Co ratios; Hf-La/Th and La/Sc-Co/Th diagrams; and rare earth element patterns) indicate that the sediments in the Baiyun Sag are mainly derived from felsic source rocks. Although the uplift around the Baiyun Sag includes multistage magmatism and basic rocks, it has little influence on the sediment in the depression, and no sediments were transported by the Han River to the Baiyun Sag. Geochemical data and detrital zircon U-Pb ages suggest that the Pearl River was formed in the Early Oligocene and became the main source for the sediments in this area. The river carried felsic material from the South China Coast forming a large delta on the northern slope of the Baiyun Sag. The ratios of Eu/Eu*, La/Sc, La/Co, Th/Sc and Th/Co, and heavy mineral data indicate that the sediments transported by the Pearl River to the Baiyun Sag were changed from the arc-dominated coastal zone to the interior of the South China Block during the Late Oligocene to Early Miocene and even reached the south-eastern margin of the Tibetan Plateau (West Yunnan Plateau). The La-Th-Sc and Th-Sc-Zr/10 diagrams suggest that the provenance of these sandstones is evolved continental island arc or an active continental margin setting.