Homogenization of zoned garnets from pelitic schists

ABsrRAcr Garnets from low-variatrce pelitic assemblages in the garnet and staurolite zones of the Mt. Rabfuh area, 8.C., generally show strong compositional zoning. A decnease in Mn oufwards from the oore is balanced by an increase in Fe and Mg. Zoning in garnets from the cordierite zone is weak or absent. Mn oommonly insreases.sligbtly from core to rim; Mg shows a corresponding decrease. Ihere is little or no Ca zoning in eitler high or low-grade rocks. At all metamorphic grades, the garnet rims are in partition equilibrium with adjacent ferromagnesian minerals. Ilmenite inclusions in low-grade rocks are in eguilibrium with immediately adjacent garqet, but inclusions in cordierite-zone ganrets are not in equilibrium with €ianet. The data suggest: (1) gxowth of zoned garnets by a fractionation-dopletion process aX lower grades; (2) homogenization of zoned garnets at temperatures abovo about 600'C; (3) modification of the bulk composition of homogeneous garnets by exchange equilibria involving garnet and cordierite; and (4) formation of Mn-rich rims on both high and low-grade garnets during retrogression of the garnet. Volume diffusion was the main process which created and modified the zoning in the cordierito zone, but was of only minor importance in lower grade rocls. The onset of significant volume diffusion may have initiated changes in the equilibria with other minerals.