Zircon U–Pb age constraint and a basement-bounding fault in Eocene pyroclastic rocks of the eastern San’in Belt, southwest Japan: Possible association with a cauldron structure

抄録

The Paleogene pyroclastic rocks in southwestern Tottori City, eastern San’in Belt, Japan, are an isolated remnant with a poorly constrained age and uncertain basement relationships. In this paper, we present lithofacies observations and a zircon U–Pb age for these pyroclastic rocks, and details of a newly identified basement-bounding fault (i.e., the Kigodani Fault). The volcanic rocks consist mainly of massive rhyolitic welded lapilli tuff, interpreted as pyroclastic flow deposits. Laser ablation-inductively coupled plasma-mass spectrometry zircon U–Pb dating of a welded lapilli tuff sample from the Kigodani area yielded a weighted-mean ²³⁸U–²⁰⁶Pb age of 36.2 ± 0.1 Ma, providing a constraint on the timing of eruption and deposition of the dated pyroclastic rock. The Kigodani Fault juxtaposes these volcanic rocks against the Sangun Metamorphic Rocks and records brittle deformation during volcanic and hydrothermal activity. Although there is no conclusive evidence for a cauldron structure, this fault provides rare outcrop-scale evidence for a basement-bounding fault associated with Eocene felsic volcanic rocks in the eastern San’in Belt.