K-AR DATING OF ILLITE-RICH ROCKS IN THE CHAMPLAIN VALLEY, VERMONT: AN INVESTIGATION OF POST-TACONIAN FAULTING AND FLUID FLOW

The main tectonic event to affect northwestern Vermont during the Phanerozoic was the Ordovician Taconian orogeny. Subsequent events in the northern Appalachians include the Devonian Acadian Orogeny, the late Carboniferous to Permian Alleghanian Orogeny, and Mesozoic extension. The purpose of this study is to perform K/Ar dating of a series of illite-rich rocks in order to constrain episodes of fluid flow and illite crystallization associated with compressional (and possibly extensional) events. Targets for K/Ar dating include (1) illites in fault clays of the Champlain Thrust, a fault which was first activated during the Taconian Orogeny; (2) illites from a Middle Ordovician K-bentonite bed on the shores of Lake Champlain; (3) illites in fault clays from a high-angle normal fault that juxtaposes Cambrian Winooski Dolostone and Monkton Quartzite; and (4) illite, illite-smectite (I/S) and authigenic K-feldspar in clay fractions from Brandon Lignite, a feature that appears to have formed via hydrothermal alteration of phyllitic layers in the Cambrian Dunham Dolostone. These samples were separated into 2-1, 1–0.5, 0.5–0.2, 0.2–0.15, and < 0.15 μm size fractions. Each fraction may represent distinct crystallization periods, as has been shown in previous studies elsewhere. The following represent preliminary K/Ar dates from various size fractions: (1) illites in Champlain Thrust fault clays range from Carboniferous (325 ± 5 Ma; 1–0.5 μm) to Late Jurassic (153 ± 4 Ma; <0.15 μm); (2) illites in the Middle Ordovician K-bentonite range from Devonian (400 ± 5 Ma; 2–1 μm) to Carboniferous (327 ± 4 Ma; <0.15 μm); (3) dates on illites from the high-angle fault are pending; and (4) illites, I/S and Kspar from the Brandon Lignite produce preliminary ages of 316 ± 5 Ma (2–1 μm) and 321 Ma ± 5 Ma (1–0.5 μm). The abundance of Devonian and Carboniferous ages suggests post-Taconian illite growth during episodes of fluid flow associated with the Acadian and Alleghanian orogenies. XRD analysis using the program MudMaster suggests multiple generations of illite growth which may correspond to renewed episodes of fault motion. The Jurassic age of the finest illites in the Champlain Thrust fault clay may relate to Mesozoic unroofing in the Northern Appalachian which has been documented by numerous apatite and zircon fission-track studies.