Metamorphic zones of the Sambagawa metamorphic belt in central Shikoku, Japan

Abstract

The four progressive mineral zones, chlorite, garnet, albite-biotite and oligoclase-biotite zones, were delineated in the whole region of the Sambagawa belt in central Shikoku using the mineral assemblages of the pelitic schists. Approximate parallelism of the metamorphic zones and lithologic boundaries shows that the isothermal surface nearly coincides with the bedding plane during the main stage of the Sambagawa metamorphism. The thermal maximum defined by the oligoclase-biotite zone is confirmed at three different structural levels, and some of the mineral zones in the structurally higher levels are partially disrupted, resulting in a more complex zonal pattern than hitherto accepted. The zonal pattern of regional metamorphism cannot be interpreted as a result of a simple large-scale recumbent fold, but rather suggests that the region consists of a pile of thrust nappes, each including rocks of different metamorphic grade and different polarity of increasing metamorphic grade. -from English summary