Relative scales of thermal- and fluid infiltration-driven metamorphism in fold nappes, New England, U.S.A. Available to Purchase

Metamorphism of the Waits River Formation WRn was studied using petrologic and stable isotopic methods in order to determine the relative importance and scales of thermal perturbations and fluid infiltration during folding and regional metamorphism at mid-crustal levels. Metamorphism of the WRF involved the growth of biotite and tremolite at the expense of muscovite and dolomite. Reaction progress in synclines is enhanced relative to anticlines and involved considerable devolatilization of CO₂. The C-O-H fluid in equilibrium with the WRF during metamorphism was an H₂O-rich binary H₂O-CO₂ mixture. Petrologic considerations yield fluid/rock ratios of between 0.01 and 1.07 in O equivalents with higher fluid/rock ratios in samples containing tremolite.

Comparison of observed values of δ¹³C_carb with calculated values based on Rayleigh distillation shows that Rayleigh distillation has affected the isotopic composition of C in carbonate. The calculations indicate that either some samples were subjected to infiltration of fluids relatively depleted in ¹³C or that heterogeneities in δ¹³C_carb existed in the protoliths.

Calculated shifts in δ¹⁸O_carb based on lever-rule effects and Rayleigh distillation are smaller than, and in some cases in the opposite direction of, observed shifts based on a protolith value of δ¹⁸O_carb = 20‰. Infiltration of an O-bearing fluid depleted in ¹⁸O relative to the calc-silicates is necessary to explain the shifts in δ¹⁸O_carb observed in these rocks. Isotopic considerations yield fluid/rock ratios in qualitative agreement with those calculated through petrologic considerations.

Mass balance considerations show that the amount of hydrous fluid necessary for fluid-present metamorphism in the calc-silicate unit could be derived from dehydration of the immediately adjacent amphibolites and pelitic schists.

Based on the distribution of metamorphic isograds and the extent of fluid/rock interaction, it is concluded that the differences in metamorphic grade between fold sets are the result of thermal perturbations on a scale of tens of square kilometers. The differences in metamorphic grade within individual folds are the result of differences in the amount of fluid infiltration that varies over a distance of tens or hundreds of meters.