T66 Different Personalities of Granites and Rhyolites: Silicic Magmas from the Lower Crust to the Surface II—A Session in Honor of Calvin Miller, Recipient of the MGPV Distinguished Geological Career Award

Sunday, 4 November 2018
1:30 PM - 5:30 PM
Room 240-241

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

Paper number: T66-12
Program: Technical Sessions
Time: 4:25 PM – 4:40 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-324752

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