Date of Award
Program or Major
Earth and Environmental Sciences
Doctor of Philosophy
Peter J. Thompson
Julia G. Bryce
The Neoproterozoic to Early Paleozoic metamorphic, deformational, geochemical, and tectonic history of the Vermont Appalachians is preserved in ultramafic, mafic, and pelitic rocks exposed west of the Iapetan suture zone in the Green Mountain and Rowe Slices of Vermont. Whole-rock geochemical analyses of polymetamorphosed amphibole-bearing mafic rocks within this zone are consistent with subalkaline basalt/gabbro protoliths that varied regionally with respect to REE concentrations inherited during crystallization. The mafic rocks that are in contact with ultramafics preserve MORB signatures, while the mafic rocks that are not in contact with ultramafics preserve REE signatures that range from MORB and OIB to signatures more enriched than OIB. Whole-rock geochemical relationships of the mafic rocks fit well into a tectonic model involving the emplacement of mafic crust that evolved geochemically through time as rift-related magmatism progressed in the Neoproterozoic during the break-up of Rodinia and the development of the Iapetus Ocean basin.
New evidence for subduction of the Iapetan Ocean basin during the Taconian Orogeny comes from the metamorphic petrology and structural relationships of an ultramafic-mafic-pelitic mélange in Stockbridge, Vermont that formed in the Taconian subduction zone. Polymetamorphosed mafic units in Stockbridge are intercalated with pelitic meta-sediment along a Taconian fabric, which is juxtaposed against fault-bounded, partially carbonatized serpentinite lozenges that are multiply-deformed. In general, the lithologic relationships and structural styles preserved in Stockbridge are similar to those preserved in subduction zone rocks of the Tillotson Peak and Belvidere Mountain Complexes farther north. New geothermobarometric estimates derived from pseudosection analyses of eclogite from the Tillotson Peak Complex and amphibolite from the Belvidere Mountain Complex show peak Taconian metamorphism at 2.5 GPa and 1.2 – 1.3 GPa, respectively.
In Stockbridge, geothermobarometric estimates derived from pseudosection analyses of mafic assemblages containing chemically-zoned amphibole indicate that peak Taconian metamorphism of the mafic units occurred between 0.8 GPa and 1.4 GPa and involved the growth of barroisite, winchite, and magnesio-hornblende. Variable amphibole chemistry coupled with interpretative changes in mineral parageneses through time describes retrograde pressure-temperature decompression paths that culminated with the growth of actinolite in the mafic rocks at or below 0.5 GPa. For the ultramafics in Stockbridge, pseudosection analyses indicate that hydration-induced peak Taconian metamorphism occurred at 2.0 GPa and carbonatization overprint accompanied decompression to 0.4 GPa, the pressure at which the ultramafics were partially chloritized. Conditions of peak metamorphism for the ultramafics in Stockbridge are consistent with a mantle wedge source, while the mafic rocks in Stockbridge were either derived from a Taconian slab or are volcaniclastic remnants of a Taconian accretionary wedge. Consistent with geochronological data from polymetamorphosed mafic rocks throughout Vermont (e.g. Laird et al., 1984), both actinolite and chlorite overprint in the mafic rocks in Stockbridge and chlorite overprint in the ultramafics are interpreted to be a result of Acadian orogenesis.
Low-viscosity channels composed of hydrated ultramafics, partially molten sediment, and partially molten crust provided exhumation pathways for the high pressure mafic and ultramafic rocks in Vermont. Lithological packages such as the Tillotson Peak Complex, the Belvidere Mountain Complex, and the Stockbridge mélange were assembled within these exhumation channels as different lithologies were exhumed from the Taconian subduction zone. Accordingly, these ultramafic, mafic, and pelitic subduction zone remnants in Vermont are ideal subjects for examining the chemical and physical dynamics of a subduction zone.
Honsberger, Ian Wiley, "Metamorphism, Deformation, Geochemistry, and Tectonics of Exhumed Ultramafic and Mafic Rocks in the Central and North-Central Vermont Appalachians" (2015). Doctoral Dissertations. 2221.