PLUTONISM ACROSS A TACONIC-ACADIAN BOUNDARY: EVIDENCE FOR VARIABLE SHALLOW EMPLACEMENT MECHANISMS IN CENTRAL VERMONT

The Richardson Memorial Contact (RMC) in central Vermont is an enigmatic feature that has been mapped in a variety of contexts along strike. This feature essentially divides units of Cambro-Ordovician age (west) from Siluro-Devonian age units (east). Where this boundary, or nearby zones, is tectonized it has been mapped as the Dog River Fault Zone. Granitoid bodies have been mapped on both the eastern and western sides of the RMC in this area and vary in outcrop-scale features and mineralogy. Pericline twinning, tartan twinning and undulose quartz are present on both sides of the contact, suggestive of localized transformation associated with pressure/temperature gradients during cooling. Igneous bodies on the east are fine-to-medium grained biotite-granitoids that host xenoliths of meta-sedimentary Waits River Formation ranging from a few centimeters to screens several meters wide. Evidence of mechanical mingling of 10’s of centimeter-scale xenoliths are restricted to within ~1 meter of pluton contact margins. Xenoliths show no clear preferred orientation. Microstructurally, myrmikitic textures are common. High concentrations of biotite along grain boundaries may be indicative of late magmatic or post-main stage crystallization hydrothermal activity. To the west of the RMC, granitoids occur as dikes that host oriented xenoliths that mimic the main foliation of the Moretown Formation. Mechanical mingling of larger xenoliths and pervasive digestion of smaller xenoliths to produce schlieren structures is common. Porphyroclasts of plagioclase feldspar, derived from adjacent country rock, floating in granite, and isolated quartz pegmatite pods that are remnants of larger host-rock pegmatites are also present. Areas of intense mingling and digestion grade into a coarse-grained biotite-granitoid within ~1 km. This gradation is interpreted to have resulted from protracted cooling, possibly associated with deeper crustal emplacement and prolonged thermal interaction with the country rock. These observations suggest different emplacement styles and potentially different emplacement depths on the east and west side of the RMC. We interpret this to be related to a structural discontinuity that acted as a zone of inherited crustal weakness that was manifested differently east and west of the RMC.