THE DYNAMICS OF TACONIAN SUBDUCTION ZONE METAMORPHISM AND EXHUMATION: INSIGHTS FROM POLYMETAMORPHOSED MAFIC ROCKS IN THE VERMONT APPALACHIANS

Conditions of polymetamorphism of mafic rocks that occur throughout central and northern Vermont and contain amphiboles that vary compositionally are constrained with geothermobarometric pseudosection analyses. Geothermobarometry of eclogite from the Tillotson Peak Complex (TPC) in northern Vermont indicates that peak Taconian metamorphism of glaucophane occurred at a maximum pressure of ~2.5 GPa, while barroisite and winchite in amphibolite and greenstone south of the TPC reflect peak metamorphism between ~1.2 GPa and ~1.4 GPa. Subsequent retrograde decompression paths are defined in these rocks by amphibole zones composed of winchite and/or magnesio-hornblende that formed between ~0.8 GPa and ~1.0 GPa and actinolite that grew between ~0.4 GPa and ~0.2 GPa. Conditions of polymetamorphism are explained with subduction zone models that involve buoyant, low-viscosity exhumation channels composed of hydrated ultramafics and partially molten crust and/or partially molten sediment. In the context of such models, the mafic rocks in Vermont are interpreted as fragments of a Taconian subducting slab that were subducted to depths greater than ~60 Km and exhumed within a channel to less than ~10 Km depth over a time period as short as ~10 million years. The intercalation of polymetamorphosed ultramafic and pelitic rocks with the various mafic slab remnants throughout Vermont is explained potentially by rapid tectonic mixing of different lithologies during channel-driven exhumation.