GEOCHRONOLOGIC CONSTRAINTS ON THE TIMING OF METAMORPHISM AND EXHUMATION OF THE TILLOTSON PEAK COMPLEX IN NORTHERN VERMONT

The Tillotson Peak Complex (TPC) in northern Vermont records high-pressure (HP) subduction zone metamorphism that occurred during the Ordovician Taconic Orogeny. Previous studies have documented a polymetamorphic history, with peak metamorphic pressures up to 2.5 GPa and temperatures of 550°C. Prior to this research, constraints on the timing of metamorphism in the TPC were limited to a ⁴⁰Ar/³⁹Ar total fusion age of 468 ± 6.4 Ma for glaucophane. This study integrates ⁴⁰Ar/³⁹Ar step heating analyses with field and microstructural observations to further constrain the subduction–exhumation history of the TPC.

Thin section analyses on samples of felsic gneiss, pelitic schist, amphibolite, and blueschist suggest deformation during varied P-T conditions. The earliest and highest-grade metamorphic event documented in our samples is preserved in a pristine blueschist, as well as in an eclogite boudin overprinted by amphibolite facies metamorphism. Inclusions of paragonite, titanite, and omphacite in garnets, locally defining S₁, suggest that some blueschists may have formed in the retrograde path in association with the S₂ foliation. A greenschist facies metamorphic overprint in most samples is also associated with S₂, primarily defined by epidote, white mica, and chlorite. Later deformation and metamorphism is recorded by microfractures in garnet and epidote and fractures in thin section that crosscut S₂, as well as chlorite pseudomorphs after garnet.

U-Pb dating of titanite and ⁴⁰Ar/³⁹Ar step heating of multiple phases yielded ages corresponding to the Taconian and Salinic orogenies. Ages of ~485–475 Ma are attributed to prograde metamorphism (M₁) and S₁ development. Ages that span ~465–455 Ma are interpreted to document retrograde M₂ metamorphism from blueschist–greenschist facies metamorphic conditions during exhumation and S₂ development. Correlation of microstructures in these samples with map-scale folds suggest that E–W trending folds developed during this time. Ages in the range of ~450–440 Ma are primarily obtained along the margins of the TPC. Younger ages approaching 420 Ma are observed locally in apparent age gradients, and are interpreted to reflect metamorphic overprinting that resulted in the chlorite pseudomorphs after garnet and the growth of albite in schist and gneiss samples.