P-T Path of Abukuma Metamorphic Rocks Inferred from Inclusions and Compositional Zoning of Garnet

Abstract

The Cretaceous Abukuma metamorphic terrane consists of the oceanic Gosaisyo Series overthrust onto the terrigenous Takanuki Series. Although the dominant mineralogy defines one of the classic andalusite-sillimanite-type progressive metamorphism, there are several lines of evidence suggesting an earlier higher-pressure history of the Takanuki Series. Garnet in the Takanuki pelitic rocks commonly shows textural sector zoning and preserves growth compositional zoning despite the high metamorphic grade, suggesting rapid and continuous changes in P-T conditions and a relatively short duration of high-temperature conditions. The common high grossular content of garnet interiors (up to ca. 30 mol%) overgrown by Ca-poor rims (less than 3 mol% grossular) in the pelitic rocks containing Al₂SiO₅ minerals, plagioclase, and quartz indicates high-pressure (> 12 kbar) and subsequent low-pressure (ca. 6 kbar) conditions. In addition, the occasional presence of low Ca cores with sillimanite-bearing plagioclase inclusions suggests earlier high-temperature and low-pressure conditions. Thus, the Abukuma (Takanuki) metamorphic rocks are inferred to have experienced rapid hightemperature loading and subsequent unloading, which may be in common with some Cretaceous metamorphic rocks in the circum-Pacific region.