Spatial distribution of garnet indicating control of bulk rock chemistry in the Sanbagawa metamorphic rocks, Kanto Mountains, Japan

摘要

Traditional metamorphic geology admitted that the appearance of specific index mineral, such as garnet, indicated the rocks experienced higher peak metamorphic condition. It is largely true, but it is still unclear what is going on in the rocks at the very start of the garnet formation. One of the key must lie in the area where apparent isograd is outcropped. Nagatoro area is located in the Kanto Mountains, and it is where the low grade metamorphic rocks of the Sanbagawa metamorphic belt are exposed. Appearance of garnet is the index of the higher metamorphic grade in this area. Several outcrops in Nagatoro area are known to contain garnet, thus being good samples of the outcropped isograd. In this study, spatial distribution of garnet within such an outcrop several tens of meters long was investigated. The samples are pelitic schists, and the mineral assemblage was basically quartz, plagioclase, muscovite, and chlorite. 36 of the 55 samples contained garnet. Garnet grains were small, most of them with diameters less than 100 μm. Most of the garnet grains were euhedral to subhedral and were found within micaceous lamellae which form the foliations of the pelitic metamorphic rocks. The micaceous lamellae were constituted mainly by muscovite with lesser amount of chlorite. Occurrence of garnet-bearing rocks within the outcrop seemed to be restricted in certain structural layers. The structural layers are known to be nearly parallel to the lithologic boundary in this area. Mapped chemical profiles of garnet revealed that the garnet grains exhibited euhedral growth of the crystal. The core part was relatively large and homogeneous, with quite a high Mn end-member (spessartine) content (X~(Sps) > 50%). Irregular shaped inner core was preserved. These features indicate that the texture preserves the beginning stage of garnet growth. The trend of chemical composition of garnet rim and adjacent chlorite is consistent with the bulk rock chemistry control. Spatial distribution of garnet, at the start of its growth, was probably controlled by the bulk rock chemistry.