USING PETROLOGIC AND STRUCTURAL EVIDENCE TO EXPLAIN A MECHANISM FOR PLUTON EMPLACEMENT ACROSS A MAJOR TECTONIC BOUNDARY IN THE CENTRAL VERMONT APPALACHIANS

Contact aureoles associated with granitic intrusions along the margin of the Taconic-Acadian interface, or Richardson Memorial Contact (RMC), in central Vermont preserve complex relationships between regional metamorphism and deformation and contact metamorphism associated with pluton emplacement styles. Studies of the Waits River Fm. along the eastern side of the RMC and the Moretown Fm. to the west, immediately adjacent to plutonic bodies, suggests different modes of emplacement. East of the RMC are dikes that intrude parallel to the main foliation (often parallel with bedding). Many granitic intrusions east of the RMC are fine to medium grained, biotite-rich, alkali-feldspar + quartz dominated. Concordant granite dikes have distinct contacts where little mingling is evident. The texture and mineralogy of the Waits River Fm. in these zones does not change significantly at contact margins, suggesting that little heat and pressure was manifested in the immediate country rock during magmatism. Further east from the RMC, exposures with evidence of mingling between the granite and country rock are more common. Country rock xenoliths are often rounded and appear to have chemically interacted with immediately surrounding magma and to have deformed in a ductile fashion. On the western side of the RMC, mechanical mingling is observed along with evidence of active mixing between xenoliths of phyllitic Moretown Fm. and granitic magmas. These features are preserved as wispy schlieren textures where country rock blocks are most thoroughly digested suggesting contamination of granite at deeper structural levels than is currently exposed. Xenolith-rich areas vary extensively in size and shape and are at a larger scale than the areas on the east side of the RMC. However, with increasing distance from the RMC to the west, more typical “pinstripe” granofels rocks of the Moretown Fm. is dominant. Closer to the RMC and along-strike to the north are exposures of homogeneous, coarse-grained, biotite granodiorite. This suggests a marked change in the crustal level exposed across as well as along strike of the RMC that may be associated with complex shearing associated with late Acadian deformation that is nearly synchronous with regional plutonism.