Provenance of rift sediments in a composite basin–mountain system: constraints from petrography, whole-rock geochemistry, and detrital zircon U–Pb geochronology of the Paleocene Shashi Formation, southwestern Jianghan Basin, central China

摘要

The Paleocene Shashi Formation, the host formation of potassium-rich brines and solid potash in the Jianghan Basin, central China, is characterized by rift sediments in a composite basin–mountain system. However, the provenance of the Paleocene Jianghan Basin remains poorly understood. A combined study of petrographic examination, geochemical analysis, and zircon U–Pb dating was conducted to characterize the provenance of the Shashi Formation. The petrographic analysis of the Shashi Formation reveals that these samples have predominantly quartz–lithic compositions with minor contributions from other minerals. Whole-rock geochemical data indicate that this formation experienced only slight to moderate alteration. The chondrite-normalized rare earth element (REE) patterns of these samples are essentially parallel to those of the upper continental crust (UCC), as their negative Eu-anomalies and Th/Sc and La/Th ratios suggest an average source composition similar to that of the UCC. These values, together with plots of their Th/U vs. Th ratios, suggest that certain samples have undergone moderate degrees of weathering and sedimentary recycling. The analysis of major and trace elements (i.e., La/Th–Hf, Th/Sc–Zr/Sc, La/Y–Zr/Sc, TiO~(2)–Fe~(2)O~(3)_(T) + MgO, Al~(2)O~(3)/SiO~(2)–Fe~(2)O~(3)_(T) + MgO) indicates that their sedimentary sources were felsic rocks from an active continental margin or a continental arc containing a minor amount of recycled sediments derived from a passive continental margin. The Shashi Formation contains detrital zircons that primarily have U–Pb ages of 2.5, 1.8 Ga, 725–965, 425–475, 310–350 and 200–260 Ma. These results reveal that the pre-Ordovician zircons are derived from recycled sediments of the Yangtze Block and thus originate from the surrounding ranges. The magmatic rocks of the South Qinling Belt and the Jiangnan Orogen are responsible for supplying the Ordovician to Triassic detrital zircons. These provenance data combined with paleocurrent analyses can be used to identify multiple source characteristics in the Jianghan Basin. During the Early Paleocene, most clastic material was derived from a southern source; during the Late Paleocene, northern and western sources contributed most of the detritus to the basin. The supply of clastic material was thus broadly variable and widely dispersed throughout the basin. This study shows that the architecture of the region and the rifting process exerted important influences on this sediment dispersal.