STABLE ISOTOPE ANALYSES OF POST-RIFTING CALCITE VEINS IN THE LAKE CHAMPLAIN BASIN: A RECORD OF TECTONIC REJUVENATION?

We report here on results of stable isotope analyses of a suite of carbonate veins hosted within lower Paleozoic meta-limestones and shales across western Vermont. Prior U-Pb age dating of veins indicate several episodes of brittle fracturing and vein formation across the region; a long period from roughly 116 to 72 Ma, and a later period from roughly 15 Ma to present. Isotopic analyses suggest calcite precipitation from distinct fluid sources and at variable temperatures for each veining episode. For example, veins with U-Pb dates ranging from 96 to 80 Ma are 13C depleted, indicating precipitation from open-system fluid, perhaps of hydrothermal origin. Oxygen isotopes suggest precipitation of these veins occurred at temperatures ranging from 100 to 120°C. This fracturing and veining episode from 96 to 88 Ma overlaps with the later stages of dike intrusion in northern New England. Later formed veins overlap with the timing of compression in southern Quebec (~72 to 5 Ma) (Faure et al., 1996) and with the timing of accelerated exhumation of the White Mountains from roughly 85-65 Ma (Amidon et al., 2016). Calculated precipitation temperatures of the open-system veins indicate formation at 2 to 3 km depth, which is broadly consistent with the exhumation history inferred apatite fission track and U-Th-He thermochronology. Veins with U-Pb ages ranging from 116 to 96 Ma have distinct isotopic compositions that may be related to host rock composition, suggesting possible precipitation from closed-system fluid sources.