Origin, stratigraphic and evolutionary significance of K-bentonites in the upper Ediacaran of South China

Abstract

Several K-bentonite beds have been discovered in the upper part of the Ediacaran system in South China. Existing researches mainly focus on the zircon U-Pb geochronology of the K-bentonites. However, the nature of the primary magmas of the K-bentonites, the tectonic setting of their source volcanoes, the significance of their geochemical correlation, and their potential implications to evolution of late Ediacaran life have not been unraveled. Here, whole-rock and zircon geochemical studies have been carried out on the K-bentonites in the top of the Miaohe Member from the Jiuqunao section, and in the basal Liuchapo Formation and the lower Dengying Formation from the Fanglong section. The major and trace element contents of the K-bentonites and the trace element contents of volcanic zircons in the K-bentonites suggest that the nature of the primary magmas of the K-bentonites from the Fanglong and Jiuqunao sections is trachyandesitic. The oxygen isotope compositions of the volcanic zircons in the K-bentonites suggest that the source magmas of the K-bentonites from the two sections are mainly from the crust. The whole-rock Y-Nb, Ta/Yb-Th/Yb, Yb-Th/Ta, and Ta/Hf-Th/Hf diagrams, and the volcanic zircon Hf-U/Yb, Y-U/Yb, Th/U-Nb/Hf, and Th/Nb-Hf/Th diagrams of the K-bentonites show that the tectonic setting of source volcanoes of the K-bentonites was a convergent environment, which may be the result of the assembly of Gondwana during late Ediacaran. The geochemical results suggest that the K-bentonites in the basal Liuchapo Formation at the Fanglong section are correlative with the K-bentonite in the top of the Miaohe Member at the Jiuqunao section. Therefore, the geochemical correlation of the K-bentonites indicate that the age of 550.5 ± 1.0 Ma for the K-bentonite in the top of the Miaohe Member may be too young to be used as the end time of the Doushantuo negative carbon isotope excursion (DOUNCE), and that the end time of this event should be older than the age 557 ± 3 Ma of the K-bentonite in the lower part of the Dengying Formation at the Fanglong section. Thus, the termination of the DOUNCE and the Shuram carbon isotope excursion (terminated at 567.3 ± 3.0 Ma) may be isochronous. As volcanic ashfall events are known to supply nutrients to the oceans, we hypothesise that the widespread volcanism of the late Ediacaran may have enhanced oceanic productivity and play a role in the evolution of life at this time.