High-Mg garnets in kyanite garnet amphibolite from the Tonaru metagabbro mass in the Sambagawa metamorphic belt, Besshi district, central Shikoku, Japan

The Tonaru mass occurs as a large lenticular body in the highest-grade portions of the Sambagawa schists in the Besshi district.The mass consists of diopside amphibolite and garnet epidote amphibolite accompanied by small amounts of eclogite.Kyanite garnet amphibolites from the western parts of the Tonaru metagabbro mass are composed of amphibole (calcic-amphibole; magnesiohornblende, tschermakite), zoisite, kyanite, garnet, phengite, paragonite, margarite, chlorite, quartz and pyrite, and show banded structure consisting of alternating amphibole and zoisite-rich layers.Porphyroblastic garnets reach up to 8 cm in diameter, and show a prograde growth zoning, with X Prp (0.40-0.52) increasing from core to rim.Magnesium contents are highest in the outermost rim (MgO 13.82 wt%), suggests the rims of the garnets record the peak metamorphic stage.Some microveins occur throughout the garnet.These microveins are filled by Fe-rich garnet, occasionally accompanied by zoisite.Micro-veins in the same garnet have an almandine-rich (X Alm 0.45-0.50,X Prp 0.25-0.34)composition.The metamorphic evolution of the kyanite garnet amphibolite from the western part of the Tonaru metagabbro mass is probably divided into two metamorphic events.The porphyroblastic high-Mg garnet represents prograde metamorphism from the epidote-blueschist facies that reached into the eclogite facies at the peak metamorphism.Fe-rich garnets developed in the microveins commonly coexist with fracture-filling minerals (kyanite, zoisite, magnesiohornblende, tschermakite and chlorite) that are evidence of a second high-P metamorphic event.This second metamorphic event probably correlates with the prograde Sambagawa metamorphism of the oligoclase-biotite zone.