Sector structure and compositional zoning of garnets from the Higo metamorphic rocks, west-central Kyushu, Japan

Abstract

Garnets with textural sector structures were found in garnet-biotite-cordierite gneisses from the Higo metamorphic belt, western-central Kyushu. The garnet contains dusty core and clear margin. The dusty core is hexagonal or octagonal in shape in thin section, and contains abundant very fine inclusions (<1 μm) of both fluid and solid phases. The fine inclusions are aligned nearly straight, being perpendicular to the outline of the dusty core. The sector structure is interpreted in terms of the rhombododecahedral model garnet that consists of twelve rhombohedral pyramids within which fine inclusions are aligned straight perpendicular to the basal surface {110}. The garnets show well developed compositional zoning (Mn- and Ca-rich core; Mg- and Ferich rim). The compositional contour lines, obtained from two-dimensional compositional mapping using an electron probe microanalyzer, are nearly perpendicular to the fine-inclusion lineations within each sector. We consider that this orthogonal relationship between the inclusion-lineation and the compositional structures is an important primary feature determined during the crystal growth of garnet, and that the isochemical contour lines (or isochemical surfaces in three dimensions) are ‘isochronal surfaces’ parallel to crystal surfaces {110} during the crystal growth. The possibility of the surface equilibrium during the crystal growth and the extent of the diffusion modification of the primary (i.e., growth) zoning are discussed.