Fluid behavior in ultramafic rocks promoted by contact metamorphism: An example from the Sanbagawa Belt, northern Akaishi Mountains, central Japan

Abstract

Contact metamorphism of ultramafic rocks provides a valuable natural laboratory for investigating fluid-assisted metamorphic reactions under well-constrained thermal conditions. In such settings, fluid behavior associated with dehydration reactions can be recorded as systematic spatial variations in mineral assemblages. In this study, we investigated a contact aureole in the Sanbagawa high-pressure metamorphic belt in the northern Akaishi Mountains, central Japan. In this aureole, the boundary between the Kifune plutonic body and the surrounding metamorphic rocks cuts almost perpendicularly across the structural arrangement of the ultramafic body. This geometry allows spatial variations in mineral assemblages to be examined primarily as a function of distance from the intrusive body. The ultramafic rocks were classified into four zones according to their proximity to the intrusive boundary: lizardite (non-contact metamorphic zone), antigorite, olivine + talc, and Ca-amphibole zones. The antigorite to Ca-amphibole zones, located within approximately 2.5 km of the intrusive boundary, were affected by contact metamorphism. Mineral assemblages from the non-contact metamorphic zone to the olivine + talc zone can be explained by metamorphic reactions in the MgO–SiO₂–H₂O (MSH) system, whereas those in the Ca-amphibole zone involve Ca-bearing components. The lower temperature limits of the antigorite and olivine + talc zones correspond to key reactions: the transition from lizardite to antigorite defines the base of the antigorite zone, whereas the dehydration breakdown of antigorite defines the base of the olivine + talc zone. The Ca-amphibole zone is characterized by the formation of Mg-hornblende associated with the dehydration of tremolite and chromium–aluminum (Cr–Al) spinel, indicating higher temperature conditions. The upper temperature limit of the olivine + talc zone is constrained to be below the anthophyllite-forming reaction, as indicated by the absence of anthophyllite. Amphibole compositions in the Kifune body indicate pressure conditions of ∼2 kbar, under which the temperature ranges are estimated as <350 °C for the lizardite zone, ∼350–500 °C for the antigorite zone, >∼500 °C for the olivine + talc zone, and >∼700 °C for the Ca-amphibole zone. Localized occurrences of olivine veins, calcic plagioclase, and Na-bearing Ca-amphibole (pargasite) were also observed near the intrusive boundary. These features, together with the discrepancy between the estimated temperatures of the ultramafic rocks and the results of conductive thermal modeling, suggest that the intrusion induced large-scale dehydration reactions within the ultramafic body and the inflow of external fluids. Such fluid influx likely played a major role in the enhanced transport of heat and chemical components.