Provenance identification based on EPMA analyses of heavy minerals: Case study of the Toki Sand and Gravel Formation, central Japan

Abstract

AbstractTo perform efficient provenance analysis, a method for quick identification of heavy minerals by using an electron probe microanalyzer (EPMA) was developed. The concentrations of 28 elements were measured in individual grains to identify the mineral species based on their chemical composition. Provenance rocks can be identified based on the abundance ratio of identified minerals and the chemical composition of specific minerals. This method was applied to samples from the Toki Sand and Gravel Formation in the Tono area, central Japan. At the sampling location, gravels of the Naegi‐Agematsu Granite and the Nohi Rhyolite were observed in the upper member, whereas gravels of only the Nohi Rhyolite were observed in the lower member. Heavy‐mineral compositions of the bedrock samples of the Nohi Rhyolite and the Naegi‐Agematsu Granite, as well as the sediments at the sampling location, were measured by EPMA analysis. Both bedrocks commonly contain ilmenite, biotite, allanite, and zircon, and the abundance ratios of heavy minerals were similar. Instead, MnO contents of ilmenite grains and Y2O3 contents of zircon grains were used as indexes to distinguish the two bedrocks. As a result, the Y2O3‐rich zircon grains (≥ 1 wt%) observed in the Naegi‐Agematsu Granite were obtained in the upper member but not in the lower member. The MnO contents of ilmenite grains were higher in the Naegi‐Agematsu Granite and the upper‐member sediments than those in the Nohi Rhyolite and the lower‐member sediments, although the difference was small probably because of Mn dissolution due to secondary alteration. These results indicate that the Naegi‐Agematsu Granite was added to the provenance of the sampling location after deposition of the lower member, which is consistent with the geological description. The change in provenance could have been caused by uplift and denudation of the Atera Mountains associated with reactivation of the nearby Atera Fault.