Pressure-temperature evolution of andalusite-kyanite-sillimanite-bearing pelitic schists from Nishidohira, southern Abukuma Mountains, Northeast Japan: Implications for Cretaceous rapid burial and exhumation in the Northeast Asian continental margin

Abstract We provide new petrological data of andalusite-kyanite-sillimanite-bearing pelitic schists from Nishidohira in the southern Abukuma Mountains, Northeast Japan. Investigation of the pressure-temperature (P–T) evolution of amphibolite-facies metamorphism in the region described the timing and tectonics of early Cretaceous metamorphic processes. The pelitic schists are composed of garnet, biotite, muscovite, quartz, plagioclase, K-feldspar, ilmenite, and graphite, in addition to Al2SiO5 polymorphs. The polymorphs occur predominantly as rounded or rhombic minerals, and are often composite aggregates. This suggests that the aggregates are pseudomorphs of andalusite (chiastolite). The andalusite, which rarely contains xenoblastic kyanite, is subidioblastic to xenoblastic, and partly replaced by kyanite, sillimanite, and muscovite. The matrix kyanite is coarse grained and porphyroblastic, surrounded by muscovite, which is in turn replaced by coarse-grained sillimanite. Fibrolite occurs as the latest phase around kyanite and sillimanite, or replacing muscovite. These textures suggest a crystallization sequence of kyanite > andalusite > sillimanite > kyanite > sillimanite > fibrolite for the Al2SiO5 polymorphs. Phase equilibria modeling of a prograde mineral assemblage (garnet + plagioclase + K-feldspar + biotite + kyanite + muscovite + ilmenite + quartz) yielded a P–T condition of 4.8–6.7 kbar and 530–570 °C. The maximum-pressure condition prior to the peak stage has been inferred as ~9 kbar (at 600 °C) based on Raman elastic geobarometry. Garnet-biotite-plagioclase-sillimanite/fibrolite-quartz assemblages yielded the peak condition of 4.5–6.9 kbar and 630–660 °C, which is within the stability field of the inferred peak assemblage (garnet + plagioclase + K-feldspar + biotite + sillimanite + muscovite + ilmenite + quartz). Our results indicate a clockwise P–T evolution with rapid burial (from the stability field of andalusite to ~9 kbar) and subsequent rapid erosion and exhumation at ~6 kbar. Electron microprobe Th-U-PbTotal dating of monazites intergrowing with the matrix biotite yielded a metamorphic age of 139 ± 24 Ma. The high-pressure amphibolite-facies metamorphism recorded in the rocks is likely associated with the thrusting of an allochthonous crustal block (including the protolith of the Hitachi metamorphic rocks) upon the Nishidohira metamorphic rocks, with serpentinite as lubricant. This process could be related to large-scale sinistral strike-slip movement and complex metamorphic processes in Northeast Asian continental margin during late Jurassic to early Cretaceous.