MICROGEOCHEMICAL VARIABILITY RESULTING FROM TRANSPRESSIONAL SHEARING ACROSS THE TACONIC-ACADIAN INTERFACE IN CENTRAL VERMONT

Late Acadian deformation, associated with discrete shear zones orthogonal to the interface between rocks of Taconic- vs. Acadian- affinity in central Vermont, suggests that unconformable Cambro-Ordovician and Siluro-Devonian rocks were modified by late-stage transpression. New microstructural and microgeochemical evidence highlights strain partitioned bands of rheologically strong quartz and minor alkali feldspar, juxtaposed with more slip-disposed zones of muscovite/chlorite domains that are overprinted by domains of Mg- or Fe-rich biotite. Well-developed quartz/alkali feldspar and muscovite/chlorite crenulation domains, which define the regional S₃ and S₄ fabrics, are sequentially overprinted by Fe- and Mg-rich biotite shear bands, bearing sigma-type spessartine – grossular porphyroblasts. Within late shear bands, multiple generations of biotite transition from Fe-to Mg-rich biotite coincident with proximity to zones of highest strain. This subtle transition provides evidence for reworking of what has been considered Acadian S₅. Within zones structurally characterized as shear bands, garnets are riddled with quartz and feldspar inclusion trails and are made up of spessartine garnet cores with grossular rims. Overgrowth of garnet and matrix phase biotite is accompanied by new quartz, alkali feldspar, and second generation subhedral biotite/chlorite within garnet pressure shadows. Along strike, but outside the highest strain zones, micro-garnets ~30 microns in size, become more almandine rich; suggesting that lower strain conditions resulted in garnet growth at unity with matrix quartz/alkali feldspar, muscovite, chlorite, and Mg-biotite. Garnet-biotite compositions reveal temperatures along zones of highest shear strain are considerably elevated above greenschist-grade of immediately adjacent rocks. Ongoing Fe/Mg exchange barometry studies for garnet-mica-feldspar suggest intensified exchange of alkaline components in zones of highest strain. Significant net-transfer reactions appear to have been active in latest Acadian time parallel to this major structural interface in central Vermont. These reactions may have enhanced localization of shear stresses to create discrete high-strain zones sub-parallel to this major structural boundary.