The Tillotson Peak Complex (TPC) in northern Vermont records high-pressure (HP) subduction zone metamorphism that occurred during the Ordovician Taconic Orogeny, and subsequent retrograde metamorphism and deformation that occurred during the Silurian Salinic Orogeny. Previous studies have documented a polymetamorphic history, with peak metamorphic pressures possibly 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 single Middle Ordovician 40Ar/39Ar total fusion age for glaucophane. This study integrates 40Ar/39Ar step heating analyses of multiple mineral phases and U-Pb dating of titanite with field and microstructural observations to further constrain the subduction–exhumation history of the TPC. Microstructural and petrologic analyses in thin section on samples of felsic gneiss, pelitic schist, amphibolite, and blueschist suggest deformation during varied P-T conditions. The earliest and highest-pressure metamorphic event documented in the TPC samples is associated with inclusions in garnet and white mica in S1 quartz microlithons. Inclusions of paragonite, titanite, and omphacite in garnets, locally defining S1, suggest that some blueschist 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. E-W trending F2 intrafolial folds are commonly rootless in outcrop, locally defined by blueschist–eclogite-facies fold noses. Kinematic indicators relative to S2 and L2 stretching lineations give a predominantly top-to-the-E shear sense. S3 crenulation cleavage development is related to folding about E-W trending F3 folds that define the map pattern of the Tillotson Peak Complex. Locally developed S4 crenulations are axial planar to the NW-trending Gilmore Antiform. Additionally, D4 deformation and metamorphism is recorded by microfractures in garnet and epidote, as well as chlorite pseudomorphs after garnet. 40Ar/39Ar step heating of multiple phases and U-Pb dating of titanite yielded ages corresponding with the Taconian to the Salinic orogenies. Ages of ~485–480 Ma are attributed to prograde–peak metamorphism (M1) and S1 development. Ages that span ~471–456 Ma are interpreted to document retrograde M2 metamorphism through greenschist to locally blueschist-facies metamorphic conditions during exhumation and S2 development. Correlation of D3 microstructures in these samples with map-scale folds suggest that E–W trending folds developed in the range of ~455–445 Ma, recorded by minimum apparent ages in the field area, and locally as plateau ages along the margin of the TPC. Younger ages ~435–405 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 actinolite, which may be related to the timing of folding about the Gilmore Antiform. Results presented here suggest the impact of Acadian retrograde metamorphism and deformation on rocks of the TPC may be less significant than previous work suggests.

The Prospect Rock Fault (PRF) is key to our understanding of the regional tectonic evolution of Vermont during the Taconic, Salinic, and Acadian Orogenies, and may have played an important role in the exhumation of blueschist and eclogite-facies rocks in the Tillotson Peak Complex (TPC) during the Taconic Orogeny. The TPC is in the footwall of the PRF in the eastern limb of the Green Mountain Anticlinorium. In the TPC, the dominant foliation is S2 and E-W trending F2 folds parallel L2 stretching lineations, which trend orthogonal to regional N-S trending folds associated with the Taconic Orogeny. The PRF itself is folded by F2 folds. Presently, there is a lack of consensus about the role of the PRF in the exhumation of the TPC, and studies have not reconciled the formation of the E-W folds and lineations to a regional model. Oriented samples and structural data were collected from the footwall of the PRF over several transects. Samples were processed into orthogonal thin sections for microstructural analyses and for 40Ar/39Ar step heating of white mica. The dominant foliations in the PRF samples were identified through microstructural analysis and correlating the age of deformation as S2 and S3. These were defined in thin section by mica and quartz microlithons, and oriented mica grains. S1, and in some samples S2, are locally preserved in some mica domains and albite/garnet inclusion trails. S4 appears as crenulations of S3, with no significant new mineral crystallization. In the field, L2 and L3 lineations are defined by mineral and quartz rods, and L4 lineations are defined as intersection lineations on S2 surfaces. 40Ar/39Ar analyses yielded plateau ages ranging from 458.6 ± 2.0 Ma to 419.0 ± 2.4 Ma (1σ). The oldest plateau ages are just slightly younger, yet concordant, with published and new 40Ar/39Ar ages from the TPC and come from the structurally highest portions of the footwall in the northern part of the study area. Virtually all apparent age spectra show age gradients. Results from this study suggest the PRF played a role in exhumation of the TPC and ages obtained are closely aligned with deformation ages constrained from 40Ar/39Ar dating in southern Quebec for the Taconic D2 and Salinian D3 deformation. These dates may aid correlatation of ages and structures regionally and further refining of tectonostratigraphic models describing southern Quebec and New England.