Online

Aiken, Cheyne

Geology

2018

M.S.

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 subductio--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 M₂ 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.