SIMS Pb-Pb dating of phosphates in the Proterozoic strata of SE North China Craton: Constraints on eukaryote evolution

Abstract

Meso-Neoproterozoic sedimentary successions in the southeastern margin of North China Craton contain a wealth of fossil and chemostratigraphic information, which aids in our understanding of early multicellular life and environment coevolution. However, a lack of reliable age constraints means the timing of these fossil occurrences and the corresponding evolutionary tempo of multicellular life remain disputed. This study aims to provide new age constraints by means of authigenic xenotime and monazite geochronology, coupled with morphological observation and mineral chemical analyses. Morphological observation and mineral chemistry allows differentiation of authigenic xenotime and monazite from detrital origins. Authigenic xenotime from the Shijia Formation, upper part of Meso-Neoproterozoic sedimentary successions, gave a SIMS Pb-Pb age of 919 ± 23 Ma, whereas authigenic monazite from the Xinxing and Liulaobei formations, lower part of Meso-Neoproterozoic sedimentary successions, produced SIMS Pb-Pb ages of 1086 ± 17 Ma and 1086 ± 120 Ma, respectively, all of which are interpreted as minimum depositional ages. The new ages suggest the crown group multicellular eukaryotes have highly evolved and diversified prior to 1.0 Ga, and confirm the use of Trachyhystrichosphaera aimika as an index fossil for the Mesoproterozoic-Neoproterozoic transitional period.