Date of Award

5-11-2025

Date Published

June 2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth & Environmental Sciences

Advisor(s)

Suzanne Baldwin

Keywords

Eclogite;Mantle Wedge;Rodingite;Serpentinite;Subduction;Taconic Orogeny

Subject Categories

Earth Sciences | Geology | Physical Sciences and Mathematics

Abstract

The study of rocks presently exposed at the surface in mountain ranges such as the Appalachian Mountains can offer insight into the evolution of orogenic and subduction zone processes over geologic time. To better understand the timing, conditions (i.e., pressure and temperature), and tectonic setting of mineral formation in former subduction settings, geologists conduct geochronology, geochemistry, and thermobarometry analyses on rocks and minerals in paleo-subduction zones. This dissertation contains three chapters that focus on serpentinization, hydrothermal alteration of mafic rocks during serpentinization (rodingitization), and high-pressure metamorphism that occurred in a subduction setting during the first phase of the Appalachian Orogeny. The first chapter uses Sm-Nd garnet geochronology to establish the timing of rodingitization and serpentinization in the northern Appalachian Mountains. The results show that multiple generations of garnet occur in serpentinite-hosted rodingites from Belvidere Mountain, Vermont. The oldest garnets formed in a subduction setting during the Taconic Orogeny (477.3±6.1 Ma) and provide a minimum estimate for the timing of serpentinization. The youngest garnets formed 70 My later in veins from low-temperature metasomatism and fluid circulation during the Salinic Orogeny (410.0±4.0 Ma). The first chapter demonstrates that Sm-Nd geochronology can successfully constrain the petrologic evolution of rodingites despite the effects of polymetamorphism and a ~70 My hiatus between different generations of garnet growth. The second chapter uses mineral and whole-rock geochemical analyses to constrain the fluid source and tectonic setting for serpentinization during the Taconic Orogeny. The serpentinites are enriched in slab-derived, fluid-mobile elements (e.g., As, Cs, and Sb) but depleted in elements such as U and Th, which are commonly enriched in seafloor serpentinites. Relict igneous olivine and spinel have high Cr/Al and Mg/Fe ratios, respectively, indicating mineral formation in a depleted mantle wedge. Together, these data suggest that serpentinization occurred in a mantle-wedge tectonic setting as opposed to an ocean-floor setting and that serpentinization was caused by slab-derived fluids. The results refine tectonic models for serpentinization during the Taconic Orogeny. The third chapter examines garnet-bearing pelitic schists in the northern Appalachian Mountains using trace-element thermobarometry (Ti-in-quartz, Zr-in-rutile) on inclusions in garnet and thermodynamic modelling to determine the conditions of garnet growth during subduction-zone metamorphism. The results show that meta-pelites from three localities in the Worcester Mountains preserve evidence for mineral growth at eclogite facies conditions up to 18 kbar and 695°C. This discovery adds to the growing number of studies suggesting that Taconic eclogite-facies metamorphism may be more widespread than previously recognized.

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