EVIDENCE FOR SYNTECTONIC INTRUSION OF THE KNOX MOUNTAIN PLUTON IN THE CONNECTICUT VALLEY-GASPÉ TROUGH, CENTRAL VERMONT

The Silurian and Devonian metasedimentary rocks of the Connecticut Valley-Gaspé trough (CVGT) were subjected to multiple fabric-forming deformational and metamorphic events throughout the course of the Acadian orogeny. Plutons intruding the metasediments have been considered post-tectonic, but microstructural studies of the intrusions and their metamorphic aureoles indicate some of these plutons may have intruded syntectonically. In this study we investigate the relationship between Acadian deformation and intrusion of the Knox Mountain pluton of central Vermont via an integrated structural and geochronological study along a transect across rocks of the CVGT. The transect begins at the western limit of the CVGT at a faulted unconformity, the Richardson Memorial Contact, and continues into the margin of the pluton in the east. Three Acadian foliations (S₁-S₃) are documented in phyllitic rocks of the CVGT. Our observations confirm previous findings that S₁ fabrics typically dominate and closely parallel original bedding, S₂ is a crenulation cleavage, and S₃ is only locally apparent. However, we have also identified localities where S₂ is associated with compositional banding in outcrop, S₃ defines a strong crenulation cleavage, and S₁ is preserved only in S₂ microlithons. Approaching the pluton margin, field and microstructural observations indicate that the Knox Mountain pluton intruded syntectonically. Evidence includes folded and boudinaged granitic dikes at the margin of the pluton, and microstructures such as flame perthite, myrmekite, deformation twins, and textures associated with grain-boundary migration recrystallization in thin sections of the granite. In addition, thin sections from the metamorphic aureole show biotite porphyroblasts that overgrow S₁ and were deformed during S₂ crenulation cleavage development. Pluton crystallization has been dated at 371 +/- 4 Ma using U-Th-total Pb dating of monazite, an age concordant with the published age of the nearby Barre Granite at 368 +/- 4 Ma from Ratcliffe et al. (2001). The timing of fabric development in the CVGT with respect to the intrusion of the Knox Mountain pluton will be constrained using ⁴⁰Ar/³⁹Ar geochronology, and will help us better understand the timing of deformation relative to intrusion and cooling during the Acadian orogeny.