A thin-section scale original inhomogeneity of bulk rock chemistry inferred from compositional zoning of garnet in the Sambagawa metamorphic rocks, central Shikoku, Japan

Abstract

The EPMA analysis of a pelitic sample from the Besshi district, central Shikoku, in the Sambagawa metamorphic belt reveals that two types of garnet porphyroblasts with different sizes and chemical compositions are present within a single thin section. One type consisting of medium -sized (d ~ < 1 mm) grains has a high Mn content at the core and shows a pyrope -rich profile. The other type is comparatively larger in size (d ~ 2 mm). The rim compositions of both types are different. The influence of the initial bulk rock chemistry on the garnet is calculated by the forward model. A comparison of the observations with the calculated results indicates that the difference between the two types of garnet grains is attributed to the initial Fe/Mg ratio of the local bulk rocks.

Two types of garnet in Sanbagawa pelitic schists along Asemigawa River, central Shikoku

“…In contrast to the current study, grains of both groups in Inui (2008) were comparable in size with the type -A grains found in this study, and none of them showed evidence of disequilibrium growth. It can be concluded that samples from the Inui (2008) report and the samples of the current study have different causes for the close associations of the two groups of garnet.…”

“…The minor overgrowth at the outermost rim of type -A garnets presumably corresponds to the second growth stage of garnet during which time the type -B garnets grew. Inui (2008) has reported studies on a sample of pelitic schist from the Sanbagawa belt, in which garnet grains with two different chemical profiles were found, and concluded that their apparent coexistence was a consequence of substantial structural shortening that made strata with different bulk composition juxtapose in a thin section scale. In contrast to the current study, grains of both groups in Inui (2008) were comparable in size with the type -A grains found in this study, and none of them showed evidence of disequilibrium growth.…”

“…Inui (2008) has reported studies on a sample of pelitic schist from the Sanbagawa belt, in which garnet grains with two different chemical profiles were found, and concluded that their apparent coexistence was a consequence of substantial structural shortening that made strata with different bulk composition juxtapose in a thin section scale. In contrast to the current study, grains of both groups in Inui (2008) were comparable in size with the type -A grains found in this study, and none of them showed evidence of disequilibrium growth. It can be concluded that samples from the Inui (2008) report and the samples of the current study have different causes for the close associations of the two groups of garnet.…”