四川盆地五峰—龙马溪组黑色页岩中钾质斑脱岩U-Pb年代学及其成因环境研究OA北大核心CSTPCD

During the Late Ordovician-Early Silurian period,not only the black shales of the Wufeng and Longmaxi formations but also many K-bentonites were deposited in South China,and the presence of many bentonites indicated that intermittent volcanism had occurred in South China at this time.However,there are still disagreements over the source of the volcanic ash that formed the bentonites.In this paper,we investigated many Wufeng-Longmaxi shale sections from the Sichuan Basin and selected the bentonites with multilayer and larger thickness as the research objects,attempting to analyze the age of the bentonites,the duration of volcanic activity,and the source of the ash by integration of field investigation,mineralogy,major and trace element concentrations,and zircon geochronology.The bentonites have bold colors,some plasticity,with associated pyrites in field.XRD characteristics show that the bentonites are composed of clay and non-clay minerals.Clay minerals are dominated by the illite-smectite mixed layer and illite,whereas the non-clay minerals include quartz,albite,calcite,dolomite,and pyrite.Their geochemical compositions are characterized by high K2O content and low TiO2 content,LREEs are slightly enriched,HREEs are poor,with a strong negative Eu anomaly,and the Chondrite-normalized REE distribution is right-skewed.Zircon U-Pb dating by LA-ICP-MS yielded ages ranging from(448±2)Ma to(440.4±5.6)Ma,which suggested that volcanic eruptions in South China lasted for at least 8 Myr across the Ordovician-Silurian boundary.Application of various chemical discrimination diagrams(Nb/Y-Zr/TiO2,Nb-Y,Y+Nb-Rb,Zr-TiO2,Hf/3-Th-Ta,Nb/Yb-Th/Yb)implies that the bentonites were mostly from intermediate acid rocks and possibly originated from an island arc environment.The fact that the layer number and thickness of the bentonites gradually decreased from north to south,combined with the current understanding of the tectonic nature of South China,indicates that the volcanic ash was probably related to the subduction and closure of the Qinling Ocean on the northern border of the Yangtze Plate in the Early Paleozoic.