Origin of K-bentonites near the Ediacaran-Cambrian boundary in South China

Abstract

Understanding the origin of volcanic material at times of environmental change and evolutionary development may help aid investigation of causal linkages between the changes and volcanic activity. One such episode of environmental change of interest is the Ediacaran-Cambrian boundary (ECB), which follows the emergence of the Ediacaran fauna, and precedes the Cambrian explosion. Owing to recent developments in geochronology, a number of K-bentonites of ECB age have been identified in strata from South China, but systematic research on their origin has not been performed. Here, we carried out mineralogical, whole-rock, and zircon geochemical studies on ECB-age K-bentonites from the middle Zhujiaqing Formation at Meishucun and the topmost Liuchapo Formation at Pingyin in South China to investigate the origin of the K-bentonites. Whole-rock major- and trace-element data, and trace-element data of volcanic zircons in the K-bentonites indicate the source magmas are felsic, whilst Hf-isotope compositions of the volcanic zircons indicate the primary source magmas likely originated from the depleted mantle, with a minor contribution of ancient crustal material. Whole-rock data and trace-element data of volcanic zircons in the K-bentonites further indicate the source volcanoes were located in a subduction-related tectonic setting. By combining our findings with previous studies of Gondwanan tectonics, we suggest the source volcanoes were most probably located in a magmatic arc setting formed by subduction of the Proto-Tethys oceanic crust towards the Iran terrane in the northwestern margin of Gondwana. Thus, the K-bentonites represent episodes of intense felsic explosive volcanism along the northwestern margin of Gondwana during the Ediacaran-Cambrian transition and have likely been transported 1000 km to the studied sites. Large-scale felsic explosive volcanism played an important role in influencing the atmospheric and oceanic environments throughout Earth's history, and the volcanic activity documented here is contemporaneous with eruptions elsewhere in the world. The tight temporal link between this activity and the dramatic environmental fluctuations and the turnover of early life evolution during the Ediacaran-Cambrian transition, leads us to propose that volcanism may have played a role in the environmental and evolutionary events of the ECB.