Genetic analysis of bentonite—Permian Wangpo shale in Liangshan, Hanzhong, South Shaanxi Province

Abstract

Fine pyroclastic materials form bentonites through subsidence, hydrolysis, and argillation in alkaline marine environments, which can further transform to K‐bentonite. Bentonites or K‐bentonites are quite popular with geologists because their formation often reflects geologic events. However, studies mainly focus on K‐bentonite, and research on ordinary bentonites in China is almost nonexistent. Recent research shows that the Wangpo shale consists of ordinary bentonites. Through thorough field investigations, indoor experimentation and analysis, and comparison of petrologic and geochemical characteristics of the Wangpo shale with K‐bentonites, this paper explores the nature and genesis of source rocks and provides a basis for the study of ordinary bentonites. The petrological study shows that the Wangpo shale is a montmorillonite vitric tuff, which is mainly composed of montmorillonite and small amounts of quartz, opal, and heulandite. These indicate that it is a typical bentonite. In addition, the rock contains an abundance of fossils reflecting a coastal–shallow sea depositional environment. We chose the elements with stable chemical characteristics for obtaining source rock characteristics, and the results showed that the Wangpo shale has a genetic relationship with neutral alkaline magma. As a boundary between the Upper Permian and the Middle Permian, the Wangpo shale has an internal connection with eruption of the Emeishan basalt in Southwest China. In combination with previous work, it can be inferred that studying the volcanic activity represented by the Wangpo shale plays an important role in studying Emeishan large igneous provinces and the end‐Guadalupian mass extinction.