TRACE-ELEMENT ZONING IN PELITIC GARNET OF THE BLACK-HILLS, SOUTH-DAKOTA

Trace element (REE, Cr, Ti, V, Y, and Zr) analysis of garnet from the garnet, staurolite, and lower sillimanite zones of an aluminous schist of the Black Hills, South Dakota, indicates that REE zoning varies as a function of grade. Garnet-zone garnet has high concentrations of RE Es, Cr, Ti, V, Y, and Zr in the cores and low concentrations in the ri ms. Profiles of heavy REEs contain inflections between the cores and r ims, which are approximately symmetric about the cores. Staurolite-zon e garnet contains cores enriched with Y and heavy REEs, which decrease toward the rim and increase again at the rim edges but to lower conce ntrations than in the cores. Cr, V, Ti, Zr, and light REE zoning is le ss pronounced than heavy REE zoning and is less symmetric about the ga rnet cores. Almandine-rich garnet of the lower sillimanite zone displa ys no major element zonation. Trace element (Ti, Cr, V, and Zr) concen trations are minimal, and the zoning is irregular and not symmetric ab out the garnet cores. Garnet from all three zones has core-to-rim Fe/( Fe + Mg) profiles that suggest garnet growth was uninterrupted with re spect to major element components and that Mn zoning formed by a fract ionation process. Analysis of trace element zoning in this garnet reve als that the major element zoning was relatively unaffected by volume- diffusion reequilibration. Trace element zonation of all samples of ga rnet is best explained by a fractionation mechanism in conjunction wit h limited intergranular diffusion and changing partition coefficients during garnet growth. Heavy REE partitioning is especially dependent o n the major element composition of garnet. This research complements p revious research by others on the use of trace elements as metamorphic petrogenetic indicators, which demonstrated the importance of bulk-ro ck composition and phase assemblage on trace element partitioning.