EVALUATING THE SCALE AND NATURE OF EQUILIBRIUM IN METAMORPHIC ROCKS USING GARNET SM-ND AGES AND COMPOSITIONAL ZONING

The spatial and temporal scales of equilibrium in metamorphic rocks are often poorly known. We know that garnet growth changes the effective composition by fractionation of Mn and other elements because only the rim of a garnet porphyroblast is in equilibrium with the matrix as zoning develops. In this case, Sm-Nd garnet ages should vary systematically from old cores to young rims which match throughout the rock and Mn contents should correlate with garnet ages. Pelitic schist from Townshend Dam, VT contains 1-3 cm garnet with concentric zoning, from Mn-rich cores to Mn-poor rims. Core and rim compositions are distinct from each other, but cores and rims are uniform from grain to grain suggesting uniform P-T-X evolution during garnet growth in the 1.21x10⁴ cm³ volume. Garnet grew during a single metamorphic event with a clockwise P-T-t path reaching peak conditions of T=585°C at P=8.8 kbar. 5 whole rock analyses indicate major and trace element, and Sm-Nd isotopic homogeneity. Trace element concentrations in garnet (LA-ICPMS & isotope dilution TIMS) indicate weak REE zoning (Sm, Gd & Dy increase to the rim) to unzoned (Nd). 38 garnet Mn contents and Sm-Nd garnet ages for concentrically sampled grains correlate reasonably well; however, 2 core, 2 mantle, and 2 rim segments have younger ages than predicted from a linear relationship. 10 garnet cores have average X_Mn=0.23-0.19 and 2σ weighted ages of 380.6±1.9 [2 excluded]. 10 garnet rims have average X_Mn=0.02-0.01 and range in age from 379.9±0.7 to 349.6±0.5. We infer that initial garnet growth began ca. 381 Ma rock wide. The majority of mantle and rim ages are compatible with equilibrium in much of the rock volume; however, anomalous ages are spatially clustered near the center. Timescales and peak temperatures are insufficient for volume diffusion to have effected ages and systematic Mn zoning indicates that large scale recrystallization is unlikely. Metamorphic textures and narrow Mn- and REE-rich garnet rims that are spatially associated with xenotime, suggest that small scale resorption/recrystallization processes could have altered some garnet rim compositions. We suggest that chemical equilibrium was generally maintained over ca. 10 Myr. of garnet growth, but localized phenomena and/or bulk rock chemical heterogeneities caused local disequilibrium.