GARNET COMPOSITONAL ZONING AND EVALUATING EQUILIBRIUM IN PELITIC SCHIST: AN EXAMPLE FROM TOWNSHEND DAM, VT

Identifying equilibrium is critical for using geochronology, thermobarometry, and forward modeling of metamorphic rocks to interpret tectonic processes. The spatial scale of equilibrium is particularly important; however, there are few quantitative assessments of equilibrium scales. Major and minor elements, and isotopic data in a 1.21x10⁴ cm³ volume of schist provide a means for evaluating equilibrium. The rocks experienced a single clockwise P-T-t path reaching a peak of T=585°C, P=8.8 kbar. 5 whole rock analyses indicate major and trace element near-homogeneity for the high Al pelite. Matrix schist isotopic analyses are ¹⁴⁷Sm/¹⁴⁴Nd=0.1184+/-0.0032 and ¹⁴³Nd/¹⁴⁴Nd=0.5119539+/-0.0000097 (N=21, 3 outliers). 1-3 cm garnet grains have concentric zoning, from Mn-rich cores to Mn-poor rims. Cores and rims are uniform from grain to grain suggesting that P-T-X conditions were similar during each increment of garnet growth. Fe, Mg, Mn, & Ca cation numbers for 9 garnet grains (N=10,370) are strongly correlated compatible with equilibrium growth. Garnet trace element concentrations indicate little (Sm, Gd & Dy increase to the rim) to no zoning (Nd). 38 Mn contents and Sm-Nd ages for concentrically sampled garnet grains correlate reasonably well; however, some segments do not fit a Mn vs. age curve. 8 garnet cores have average X_Mn=0.23-0.19 and a 2σ weighted age of 380.6±1.9 Ma [1 excluded]. 15 garnet mid-sections have a 2σ weighted age of 377.1±1.4 Ma [1 excluded]. 12 garnet rims have average X_Mn=0.02-0.01 and a 2σ weighted age of 376.3±1.0 Ma [4 young ages excluded]. Therefore, initial garnet growth began ca. 381 and continued to ca. 376 Ma rock wide. The majority of 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 intracrystalline diffusion and systematic Mn zoning indicates that large scale recrystallization is unlikely. Metamorphic textures and narrow Mn- and REE-rich garnet rims associated with xenotime, suggest that resorption/recrystallization processes altered some garnet rims. We infer 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.