OUTCROP-SCALE VARIATION IN GARNET GROWTH ZONING: CONTROLS ON LENGTH SCALES OF CHEMICAL HOMOGENEITY AND EQUILIBRATION AT PASSO DEL SOLE, CENTRAL SWISS ALPS

A variety of disparate garnet growth zoning patterns is present on the scale of a single outcrop in gneisses from the northern boundary of the Lucomagno nappe at Passo del Sole, Switzerland. Unusual concentric and in some cases oscillatory fluctuations in Ca, Mn, and Y overprint the regional metamorphic zoning signature, and appear or disappear along strike within single layers on the meter-scale, yet may be chemically and texturally correlated amongst porphyroblasts within an individual hand sample, indicating that length scales of equilibration are greater than centimeter-scale. Locally aberrant plagioclase compositions, changes in bulk composition implied by fluctuations in garnet isopleths produced by thermodynamic models, and the correlation of the unusual garnet-zoning patterns with increased strain rates all lend support to the hypothesis that these rocks were affected by open-system infiltration of channelized fluids along an adjacent shear zone during prograde Alpine metamorphism. To test this idea, in-situ oxygen isotope analyses using SIMS were performed on three samples in an attempt to quantify changes in oxygen-isotopic garnet-water equilibrium fractionations during garnet growth. The data show that changes in major-element zoning patterns in garnet correlate with small shifts in isotopic composition that exceed the effects associated with changes in garnet chemistry or increasing temperature, supporting the interpretation that garnet zoning reflects fluid-driven shifts in the rocks' bulk chemistry.