Garnets in a Biotite Schist from Southwestern Part of Mikawa Plateau, Central Japan

The garnets in the analysed biotite schist consist of two types, euhedral garnets showing equiaxial euhedral shape and anhedral garnets showing ellipsoidal shape whose longest axes are oriented parallel to the schistosity. The euhedral garnets have inclusion-rich radial zones separated by inclusion-poor sectors (or sectors of clear garnet) which are oriented along the rational crystallographic directions, while the ellipsoidal garnets have homogeneously distributed inclusions which are oriented parallel to the schistosity in their matrix. The euhedral garnets are rarely included in the ellipsoidal garnets, suggesting that the latter postdates the former. Antipathetic relation in zonal variation between MnO and FeO are found in the garnets of both types. In the euhedral garnets normal zoning and reverse zoning develop in the cores and outermost margins respectively, while in the ellipsoidal garnets zonal variation (reverse zoning) developes only in the outermost margins. It has been assumed that, during the phase of appearance of the ellipsoidal garnets, their nucleation and growth occurred independently upon the pre-existing euhedral garnets and the overgrowth of the latter hardly occurred and that the appearance of the ellipsoidal garnets began during the phase of growth of the parts of euhedral garnets at which (= near the rim) MnO is minimum.