Characterizing rates of Acadian metamorphism using Sm-Nd geochronology and major element diffusion in garnet

Garnet, a robust, rock-forming mineral, is often used to understand metamorphic processes through study of chemical zonation, mineral inclusions, and radiometric dating. Natural sample suites in which two distinct populations of garnet are found can be particularly insightful in unraveling the pressure-temperature-time (P-T-t) evolution of rocks through Earth’s crust. We studied two sample sets from the Smalls Falls formation (northern New Hampshire, USA) and the Hawley formation (western Massachusetts, USA) which both contain a high crystallization density coticule garnet quartzite (~10⁶ crystals/cm³) and a lower crystallization density garnet schist (<10² crystals/cm³). Classical and elastic thermobarometry applied to both sample sets indicated that coticule and schist garnet may have crystallized at P-T conditions within uncertainty of each other. We collected ID-TIMS Sm-Nd isotopic data from these sample sets to better understand their nucleation history. In the Hawley formation, coticule garnet grew at 395.3±4.5 Ma (MSWD=1.6), while schist garnet from the same outcrop yielded an Sm-Nd isochron age of 376.6±4.0 Ma (MSWD=9.1). Smalls Falls formation coticule garnet yielded an isochron age of 372.4±2.1 Ma (MSWD=0.83), while schist garnet from the same formation crystallized at 367.0±1.4 (MSWD=1.3). These data indicate that spessartine (Mn) rich coticule garnets grew 18.7±6.1 Ma and 5.4±2.5 Ma before lower crystallization density schist garnets in the Hawley and Smalls Falls formations, respectively. We performed Mn diffusion modelling using FORTRAN program GarDiffMoveRim on Mn-rich small-radius garnets from the Hawley formation and found that at inferred temperatures of nucleation and growth (600°C), presently observed Mn zoning profiles persist for remarkably short time scales (<100 ky). Even at lower temperatures (500-550°C) Mn poor mantles in coticule garnets persist for <5 my. Preservation of bell-shaped Mn zoning profiles in Hawley formation garnets does not allow for residence at elevated T on the long timescales that may be inferred from our Sm-Nd isotopic data (~18 my). Instead, our Sm-Nd geochronology combined with Mn diffusion modelling suggests that heat sources during Acadian metamorphism may have been highly transient in nature, spurring garnet growth and followed by rapid cooling.