Chemical zoning of pegmatite garnets from the Ishikawa and Yamanoo areas, northeastern Japan

Abstract

Three coarse-grained garnets from granitic pegmatites in the Ishikawa and Yamanoo areas, northeastern Japan, were traversed by an electron probe microanalyser (EPMA). Garnets occur in the feldspar zone (Ishikawa) and in the muscovite-feldspar-quartz zone (Yamanoo). The three garnet grains display distinct chemical zonings one another. One garnet from Ishikawa is unzoned. In another garnet from Ishikawa and one garnet from Yamanoo, Fe and Mg contents increase from core to rim as Mn content decreases. In the two zoned garnets, Ca content either monotonously increases to the rim or increases then decreases. These patterns were modeled by assuming varying values for partition coefficients, associated solid-phase compositions, and final proportions of garnet crystallized from the melt. The Ca peak is reproduced if partition coefficients change during the garnet growth and crystallization of a Ca-mineral such as feldspar removes Ca from the melt. The different zoning patterns in garnet within a single pegmatite zone are largely attributable to the difference in the growth rates of garnet and co-crystallizing minerals.