MAGMATIC MICROTEXTURES IN MID-CRUSTAL, LATE SYNTECTONIC PLUTONS: A TOOL FOR UNDERSTANDING STRAIN LOCALIZATION, CENTRAL VERMONT APPALACHIANS

Thermobarometric estimates of emplacement depth for calc-alkaline bimodal plutons along the Taconic-Acadian interface in central Vermont suggest a non-linear heritage for emplacement dynamics. Plg-Hbl thermobarometery indicates emplacement pressures (P) of 4.5-5 Kb (15-17 km depth) and temperatures (T) between 550 & 600°C, generally matching interpretations of previous regional workers. However, this range skews toward higher T and P in the pluton center and to much lower T and P nearing pluton margins. Disequilibrium dihedral angles of Qtz-Plg grain boundaries suggest disruption of fractional crystallization. Lack of textural equilibrium in the pluton center, and outcrop-scale evidence of mechanical felsic-mafic magma mingling, suggest pulses of relatively hot, volatile-poor mafic magmas into a felsic chamber resulted in either melt de-polymerization or variation in undercooling rates due to volatile migration. These zones include concentrations of K-spar phenocrysts with narrow Na mantles that lack twinning, indicating less-hydrous magmas, that are often associated with high concentrations of Tschermakite-bearing mafic enclaves. Toward pluton margins, anomalously coarse, anhedral clusters of optically continuous Qtz, without shape- or crystallographically-preferred orientations, indicate prolonged late-stage nucleation. Qtz textures in these marginal zones contrast with twinned K-spar that are relatively fine-grained and euhedral with clear shape preferred orientations, but lacking any optical or chemical zoning. 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. In some cases, skeletal Bt is present as pseudomorphs after Hbl. Textures nearest the pluton margin may be associated with the disruption of undercooling due to volatile migration, induced by atypical T and P gradients, associated with injections of mafic magma or assimilation of phyllosilicate-rich host rocks. The generation of magmatic fluids by repeated infusions of heat may have created elevated vapor pressures for the mid-crust in this region suggesting the potential for elevated pore-fluid pressures having played a regional role in strain localization during latest Acadian tectonism.