NEW STRUCTURAL FINDINGS AND 40Ar/39Ar WHITE MICA AGES IN THE CLASSIC BARROVIAN SEQUENCE IN SOUTHEASTERN NEW YORK

Field mapping and ⁴⁰Ar/³⁹Ar dating of white mica (WM) across the classic metamorphic gradient in Dutchess County, New York, indicate that post-peak metamorphic and structural events add additional complication to the interpretation of this as an intact gradient, as previously noted by Ratcliffe, Buden, and Burton, 1985. Oriented samples were collected and fabrics were measured along two ~7 km transects from chlorite to sillimanite zone. These transects extend from west of Clove Mountain (chlorite zone) to the Swamp River Valley (sillimanite zone) in eastern Dutchess County. Field and petrographic analyses reveal three pervasive foliations (S₁, S₂ and S₃). S₁ slaty cleavage is folded in the lower grade rocks of the sequence, but is increasingly overprinted at garnet and higher grades by a penetrative foliation, S₂, that developed at or near peak conditions. S₂ dips steeply to the SE, is axial planar to folds in S₁, and is spatially associated with thrust zones (Ratcliffe et al, 1985). S₂ is overgrown by porphyroblasts of Bt, Bt + Grt ± Str ± Crd ± Sil, at respective metamorphic grades, that appear to approximate peak temperature conditions during or near the time of S₂ formation. S₁, S₂, and peak temperature conditions can be bracketed as late Taconic based on a depositional age of ~460 Ma (Potter, 1972, Tucker et al., 1990) for autochthonous sediment and a staurolite age of ~454 Ma (Lanzirotti and Hanson, 1997) from kyanite-sillimanite grade rocks. S₃ is a low-grade fabric composed of muscovite ± biotite ± chlorite that truncates S₂ in garnet through sillimanite grade rocks in the eastern part of the study area. S₃ dips gently to the S-SE and becomes more penetrative to the east, such that in the garnet zone, S₂ is overprinted by a weakly developed spaced S₃ cleavage, while in many samples in the sillimanite zone S₂ micas are completely overprinted by S₃ micas. S₃ is observed wrapping around and truncating peak temperature porphyroblasts. Twenty samples of lepidoblastic WM (with varying proportions of S₁, S₂, and S₃) from biotite through sillimanite zone all produce ⁴⁰Ar/³⁹Ar age spectra suggesting closure ages of 380-390 Ma. These WM ages are interpreted to define a window of cooling through WM closure (~350°C) for the study area.