BIMODAL MAGMATISM AND COUNTRY ROCK ANATEXIS ASSOCIATED WITH STRAIN LOCALIZATION IN CENTRAL VERMONT

Granite emplacement mechanisms and relative concentrations of partially digested country rock xenoliths vs. zones of bimodalism vary across the Taconic/ Acadian interface in central Vermont. Calc-alkaline bimodal plutons west of this N-S trending structural boundary preserve pressures of 4.5-5 Kb (15-17 km depth) and temperatures between 550 & 600°C based on Plg-Hbl thermobarometery. This is in contrast with granitoids of nearly identical age east of the Taconic-Acadian interface that preserve pressures closer to 3.5Kb and temperatures of approximately 450°C. Isolated zones of high strain sub-parallel to the Taconic-Acadian interface preserve upper-greenschist to lower-amphibolite facies fabrics. Concordant granite dikes in eastern areas have sharp contacts and larger bodies are fine-to-medium grained and Bt-rich with limited concentrations of pelitic host-rock xenoliths. Myrmikitic textures and high concentrations of Bt along Qtz-Ksp grain boundaries may indicate post-main stage crystallization, possibly related to late hydrothermal activity. Granitoids that intrude rocks of Taconic-affinity (west) preserve evidence of extensive digestion of smaller xenoliths to produce schlieren. K-spar phenocrysts preserve narrow, untwinned, Na-rich mantles often associated with tschermakitic mafic enclaves that are interpreted to indicate a less-hydrous magma. Pluton margins preserve anhedral clusters of optically continuous Qtz indicate prolonged late-stage nucleation. Disequilibrium dihedral angles of Qtz-Plg grain boundaries suggest disruption of fractional crystallization. Dentritic Plg (An22-26) is more common nearest pluton walls where it is networked with Qtz + Hbl aggregates to form comb layering textures, especially adjacent to mafic enclaves. Textures may be associated with the disruption of undercooling due to volatile migration driven by: 1) injections of mafic magma or 2) assimilation of phyllosilicate-rich host rocks. Magmatic fluids may have created elevated vapor pressures that led to high pore-fluid pressures leading to strain localization during latest Acadian tectonism.