Large Contact Aureole Overprinting the Ryoke Regional Metamorphic Zoning: Insights From the Takato–Komagane Area, Central Japan

The Ryoke metamorphic belt records the conditions of the middle–upper crust during Cretaceous crustal deformation and magmatism. To understand the development and evolution of the middle–upper crust, it is important to investigate the thermal structure of the Ryoke metamorphic belt. The Ryoke metamorphic rocks crop out widely in the Takato–Komagane area in Nagano Prefecture, central Japan. However, the spatial variations in metamorphic conditions in both areas are still unclear. We therefore investigated the metamorphic zoning and conducted Raman spectroscopy of carbonaceous material (RSCM) thermometry, chemical analyses, and pseudosection modeling of the Ryoke metamorphic rocks. The Bt, Sil–Kfs, and Grt–Crd zones in the Takato area, and the Bt, Kfs–Crd, and Grt–Crd zones in the Komagane area were redefined based on the mineral assemblages in metasedimentary rocks. The metamorphic zoning is different between the Takato and Komagane areas. We revealed that sillimanite occurs throughout both areas, although its occurrence changes gradually from in the matrix to inclusions in cordierite, between the Takato and Komagane areas. Peak P–T conditions of the Sil–Kfs zone are estimated to be 630°C–650°C and 320–380 MPa using RSCM thermometry and pseudosection modeling. Pseudosection modeling, microscopic observations, and thermometry results indicate higher temperatures in the Komagane area than in the Takato area. It is interpreted that part of the regional metamorphic belt underwent thermal overprinting induced by a late-stage pulse of the gneissose Ryoke granitoids, resulting in the formation of a metamorphic zone distinct from that in the Takato area, characterized by the occurrence of sillimanite inclusions.