THE RELATIONSHIP BETWEEN MAGMATISM, DEFORMATION, AND METAMORPHISM DURING THE ACADIAN OROGENY: A CASE STUDY FROM THE KNOX MOUNTAIN PLUTON, GREEN MOUNTAINS, 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 as the Iapetus Ocean closed and Avalonia was accreted onto the margin of Laurentia. 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 syn-tectonically. In this study we investigate the relationship between Acadian deformation and 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, flame perthite, deformation twins, and textures associated with grain-boundary migration recrystallization. In addition, biotite porphyroblasts in the metamorphic aureole overgrow S₁ and were deformed during S₂ crenulation cleavage development. The timing of fabric development in the CVGT with respect to the intrusion of the Knox Mountain pluton will be constrained using U-Pb and ⁴⁰Ar/³⁹Ar geochronology. The results of this work may provide further insight into the relationship between the intrusion and strain partitioning, and will help further constrain the duration and pulses of the Acadian orogeny.