2009 Volume 115 Issue 3 Pages 101-121
The Sambagawa belt is a typical high-P/T metamorphic belt. Several tectonic blocks of various size, protoliths, and metamorphic histories are scattered in the high-grade portions of the belt in the Besshi district of central Shikoku, Japan. The Gazo mass is a tectonic block located at the northeastern margin of the largest tectonic block, the Iratsu metagabbro mass. The Gazo mass consists mainly of pelitic and basic schists with intercalated layers and lenses of eclogite within layers of basic schist. Typical eclogite in the Gazo mass consists of garnet, clinopyroxene, epidote, phengite, paragonite, and quartz. Four stages of metamorphism are distinguished based on eclogite texture: (1) Pre-peak metamorphism at conditions of the eclogite facies, as indicated by inclusions in garnet and clinopyroxene. The inclusions include clinopyroxene (XJd=0.07-0.35), epidote, phengite, Na-Ca amphibole, quartz, titanite, and rutile. Garnet and omphacite crystals contain resorbed cores and overgrown rims, suggesting the occurrence of a substage of decreasing pressure and temperature or H2O infiltration during prograde metamorphism. (2) Peak metamorphism at conditions of the eclogite facies (12-25 kbar, 510-580°C), with the coexistence of garnet and omphacite (XJd=0.20-0.51). (3) Retrograde metamorphism at conditions of the epidote-amphibolite facies, producing symplectitic aggregates of clinopyroxene (XJd=0.01-0.11) + albite ± Na-Ca and Ca amphibole after omphacite. (4) Sambagawa prograde metamorphism at conditions of the epidote-amphibolite facies, producing porphyroblastic amphibole after omphacite of the eclogitic stage, with similar chemical compositions and zoning patterns to those of amphibole in the Sambagawa basic schists.
We propose that the Gazo mass became united with the Iratsu mass during exhumation of the latter, which had undergone high-P/T metamorphism at much deeper levels in the subduction zone. The sutured Gazo-Iratsu mass was then emplaced into the Sambagawa schists, followed by prograde metamorphism of the entire rock mass.
