Glaucophanic amphibole in the Seba eclogitic basic schists, Sambagawa metamorphic belt, central Shikoku, Japan : implications for timing of juxtaposition of the eclogite body with the non-eclogite Sambagawa schists

Abstract

The Seba eclogitic basic schists in the Sambagawa metamorphic belt of the Besshi district in central Shikoku, southwest Japan, preserve evidence of two distinct metamorphic events. These are a first eclogitic metamorphic event, and a second glaucophane-barroisitic metamorphic event. The peak metamorphic conditions of the first eclogitic metamorphic event are deduced from the mineral assemblage of schistosity-forming minerals of the eclogite facies (T=610-640℃ and P=12-24kbar; Aoya 2001). The peak eclogitic conditions were followed by a clockwise decompression P-T path, with retrogression into the epidote amphibolite facies at T=400-535℃ and P=6-7kbar. These conditions were obtained from phases replacing the minerals of the peak metamorphic conditions. The second glaucophane-barroisitic metamorphic event is defined by distinctly zoned amphiboles which suggest a prograde path from the epidote-blueschist facies to the epidote amphibolite facies. The prograde path is similar to that of surrounding non-eclogitic Sambagawa schists. The Seba eclogitic basic schists were juxtaposed with non-eclogitic Sambagawa schists during the prograde metamorphism of the non-eclogitic Sambagawa schists. These amalgamated metamorphic sequences subsequently underwent prograde metamorphism reaching the epidote-amphibolite facies, and were then exhumed to shallow crustal levels.