STABLE ISOTOPES IN STRAIN FRINGES FROM THE TACONIC MOUNTAINS, VERMONT

Stable isotope and fluid inclusion analyses were conducted on cm-scale quartz and calcite fibrous strain fringes from the Hatch Hill Formation in the Taconic Slate Belt of Western Vermont. The goal of this study is to assess the geochemical and thermal histories of fluids that moved through the rocks during strain fringe formation (cf. Goldstein et al., 2005 Geology). The fluids precipitated calcite and quartz in the form of fibrous strain fringes around rigid sub-spherical pyrite framboids. Three strain events are recorded in the strain fringes by three different growth directions of mineral fibers. Primary aqueous fluid inclusions in quartz have T_h ranging from 110°C to 160°C; T_m range from 0°C to -2.4°C. Values of δ¹³C in calcite from two strain fringes vary from -3.4 to -3.2‰ (n=22), with lower values found in the youngest (last formed) calcite. Values of δ¹⁸O_SMOW of calcite range from 16.0 to 17.1‰, from oldest to youngest parts of the strain fringe, while δ¹⁸O_SMOW of quartz varies from 19.0 to 20.3‰. Stable isotope thermometry of the strain fringe growth ranges from 190 to 300ºC, and demonstrates a prograde thermal history for this slate unit during strain fringe development. The δ¹⁸O_SMOW of water calculated from equilibrium considerations is in the range of 6.0 to 8.0‰, similar to metamorphic fluids in other low-grade terranes