In-situ U–Pb dating of monazite, xenotime, and zircon from the Lantian black shales: Time constraints on provenances, deposition and fluid flow events

Abstract

The Lantian formation, located in eastern South China block (SCB), forms the lower sequence of the Ediacaran period, at the transition from Proterozoic to Paleozoic. The fossils of Lantian formation have been proposed to predate the Avalon biota (about 579–565 Ma), the earliest known fossil assemblages with complex morphologies, based largely on stratigraphy. Mineral chemistry and in-situ U–Pb dating of monazites, xenotimes, and zircons from the black shale in fossil-rich zone are studied for the first time to provide critical age constraints for provenance, early diagenesis and fluid flow events. Detrital zircons and monazites give age populations similar to that of the Doushantuo formation, indicative of the same provenances for Ediacaran basins throughout the SCB. The low-Y monazites formed during early diagenesis yield a weighted mean age of 612 ± 29 Ma (n = 9, MSWD = 3.1). Age spectrum of analyzed monazites demonstrated that the sedimentation started from at least around 645 Ma. Subsequently, the overprinting events could be recorded by xenotimes and zircons precipitated from fluids during late diagenesis. Age results from xenotimes ranging from 441 Ma and 585 Ma were contemporaneous with the timing of collision of SCB with Gondwanaland, and a weighted mean age of 394.8 ± 3.7 Ma (n = 23, MSWD = 2.8) obtained from diagenetic zircons could be correlated to the post-tectonic granites widespread on the SCB. The pre-existing monazites generally remain pristine, and the multi-staged fluids were locally interacted with the samples to form Th-rich domains in monazites.