6 Petrologic Insights on Modern and Ancient Plate Margins I: The Volcanic, Plutonic, and Metamorphic Records

Sunday, 18 March 2018
8:00 AM - 12:00 PM
University Amphitheater

LINKS BETWEEN MAGMATISM AND STRAIN LOCALIZATION ALONG THE TACONIC/ ACADIAN INTERFACE IN CENTRAL VERMONT

Paper number: 6-1
Program: Theme Sessions
Time: 8:00 AM – 8:20 AM

Authors

Abstract

Variability in granite emplacement styles, east and west of the Taconic/ Acadian interface in central Vermont, suggest that this structural discontinuity acted as a zone of inherited crustal weakness that effected magma mobilization. Emplacement depth for calc-alkaline bimodal plutons west of the Taconic-Acadian interface, based on Plg-Hbl thermobarometery, indicate pressures (P) of 4.5-5 Kb (15-17 km depth) and temperatures (T) between 550 & 600°C. P/T conditions, using Ti in Bt thermometry and Bt/Msc barometry, from granitoids to the east are closer to 3.5Kb and 450°C. Intrusions into Acadian-affinity (east) units are often associated with dikes that intrude parallel to the main regional foliation (often bedding-parallel). Concordant granite dikes have sharp contacts. Larger igneous bodies are fine-to-medium grained and Bt-rich with limited concentrations of meta-sedimentary xenoliths, restricted to ~1 meter from contact margins. Myrmikitic textures and high concentrations of Bt along grain boundaries indicate post-main stage crystallization hydrothermal activity. In contrast, granitoids that intrude rocks of Taconic-affinity (west) preserve evidence of extensive mechanical mingling of larger xenoliths and pervasive digestion of smaller xenoliths to produce schlieren. K-spar phenocrysts with narrow, untwinned, Na-rich mantles are often associated with tschermakitic mafic enclaves, indicating less-hydrous magmas. Near pluton margins, anomalously coarse, 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. Pulses of relatively hot, volatile-poor mafic magmas appear to have resulted in either: 1) melt de-polymerization or 2) variation in undercooling rates due to volatile migration. The generation of magmatic fluids by repeated infusions of heat may have created elevated vapor pressures, especially immediately west of the Taconic-Acadian interface leading to strain localization during subsequent deformation along this temporal interface.

Geological Society of America Abstracts with Programs. Vol. 50, No. 2

doi: 10.1130/abs/2018NE-311308

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