EXHUMATION OF THE TILLOTSON PEAK COMPLEX IN NORTHERN VERMONT

The Tillotson Peak Complex (TPC), located in northern Vermont, records high- to ultra-high pressure metamorphism that occurred during the Taconic Orogeny (~470-440 Ma). Previous studies documented a polymetamorphic history due to the Taconic–Acadian orogenies, with peak Taconian pressures approaching coesite-eclogite facies conditions. This study integrates ⁴⁰Ar/³⁹Ar analyses with structural and microstructural data from the TPC with published literature, maps and field guides to further constrain the subduction–exhumation history.

Lithologies of the TPC include felsic gneiss, Hazen’s Notch Schist, ultramafic units, and blueschist and eclogite intercalated with pelitic schist. Lithologic layering defines S1 in the TPC, suggesting a common subduction–exhumation history. Observations to date suggest variable kinematics of deformation as function of metamorphic grade. For example, a sample of blueschist from an outcrop in Eclogite Brook preserves a dominant foliation defined by glaucophane, with lesser amounts of epidote, white mica, and titanite. The matrix foliation is continuous with an internal foliation in garnets. Rotation of garnet and its internal foliation with respect to the matrix suggests top-to-the-NW sense of shear. In contrast, a nearby sample of felsic gneiss has a dominant foliation defined by white mica, chlorite, and epidote. In this sample, chlorite tails on garnets and chlorite pseudomorphs after garnets indicate deformation during retrograde greenschist-facies metamorphism. The sigmoidal geometry of the garnets porphyroclasts suggest a top-to-the-NE sense of shear. Static recrystallization is also locally evident in samples throughout the field area as indicated by the growth of epidote at a high angle oblique to S₁, grain boundary area reduction observed in quartz-rich compositional bands, and the decussate growth of white mica.

Multiple phases (e.g. white mica, tourmaline, amphiboles) have been selected from individual samples for ⁴⁰Ar/³⁹Ar step-heating analyses. Obtaining radiometric ages on multiple phases from the same sample as well as in different microstructural contexts will help constrain the timing of peak metamorphism as well as the timing of deformation and cooling during exhumation.