Post-Taconic tilting and Acadian structural overprint of the classic Barrovian metamorphic gradient in Dutchess County, New York

Abstract

Field mapping and ⁴⁰Ar/³⁹Ar age spectrum dating of white mica across the classic metamorphic gradient in Dutchess County, New York indicate that post-peak metamorphic and structural events complicate the interpretation of this sequence as an intact Taconic metamorphic field gradient. Oriented samples were collected and fabrics were measured along two ∼7 km transects. These transects extend from west of the biotite zone (west of Clove Mountain) to the sillimanite zone (Swamp River Valley) in eastern Dutchess County. Field and petrographic analysis reveals three pervasive foliations (S1, S2 and S3). S1 is folded in the lower grade rocks of the sequence, but is increasingly overprinted at garnet and higher zones by a penetrative foliation, S2. S2 dips steeply to the SE and is axial planar to folds in S1. S2 is overgrown by porphyroblasts of biotite, biotite + chloritoid, biotite + garnet ± staurolite ± kyanite ± sillimanite, at respective metamorphic grades, that appear to approximate peak temperature conditions. S1, S2, and peak temperature conditions are constrained as late Taconic based on a depositional age of ∼460 Ma (Potter, 1972) for the Walloomsac Formation and a staurolite age of ∼454 Ma (Lanzirotti and Hanson, 1997) from kyanite-sillimanite grade rocks. S3 is a greenschist facies foliation composed of muscovite ± biotite ± chlorite that truncates or overprints 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 staurolite and higher metamorphic zones S2 micas are significantly overprinted by S3 micas. S3 is observed wrapping around and truncating peak temperature porphyroblasts. Three samples of lepidoblastic white mica (with varying proportions of S1, S2, and S3) from the biotite through staurolite zone all produce ⁴⁰Ar/³⁹Ar age spectra suggesting closure ages of ∼385 Ma. These white mica ages are interpreted to represent the time of cooling through white mica closure (∼350 °C). The ⁴⁰Ar/³⁹Ar ages in conjunction with field mapping and petrographic analysis suggest that S3 developed in the Devonian Acadian orogeny. Furthermore, the ⁴⁰Ar/³⁹Ar data constrains the timing of tilting and uplift of the Ordovician Barrovian sequence from the Middle Silurian to Early Devonian. This study suggests mid-crustal wedging during the Salinic orogeny as a possible mechanism for tilting the rocks of this region and exposing the Taconic-aged Barrovian field gradient at the surface today. We conclude the low-grade Acadian metamorphic overprint of these rocks involved deformation that produced a new (third) cleavage that was coincident with regional cooling from peak Taconic metamorphic conditions and thus required strain but no reheating during the Acadian.