Temperature-time development in the Wepawaug Schist

Recent research (Baxter et al. 2002) in the Barrovian type-locality (Scotland) has shown that peak metamorphic temperatures there were achieved at the same time across several different grades. This result is inconsistent with the crustal overthickening model postulated by England and Thompson (1984), wherein rocks which reach different metamorphic grades must have reached peak temperatures at different times. Instead, the presence of additional sources of heat, such as magmatic intrusions and metamorphic fluids, may have contributed to the contemporaneous development of low and high grades. The Wepawaug Schist, a metapelitic schist in south-central Connecticut exhibiting a Barrovian sequence, provides another terrane in which to test the model of advective heat transport. Here, we present the first Sm/Nd garnet ages from the Wepawaug Schist. HF-partial dissolution techniques were employed to cleanse the garnets of problematic mineral inclusions to ensure accuracy and improve the precision of garnet ages. Two-point whole rock-garnet isochrons yield an age of 379.2 ± 6.7Ma (2σ) for peak metamorphism in a sample from the garnet isograd, whereas garnet rim growth was dated at 379.9 ± 6.8Ma (2σ) in a kyanite zone sample. These ages are indicative of Acadian-age metamorphism. The two garnet ages are statistically indistinguishable, suggesting that peak garnet growth (and likely peak metamorphic temperatures) was attained penecontemporaneously across all metamorphic grades in the Wepawaug. A coincidence of peak metamorphic ages requires an additional source of heat to steepen the geotherm such as could be provided by local igneous intrusions and enhanced by fluid flow.