ALLEGHENIAN STRAIN LOCALIZATION AND RECRYSTALLIZATION IN THE EAST DERBY SHEAR ZONE, CONNECTICUT

The East Derby Shear Zone (EDSZ) in SW Connecticut localizes Alleghanian dextral transpressive strain along a contact between Acadian metamorphosed kyanite-sillimanite grade orthogneisses to the west and staurolite and lower-grade schists to the east. Regional biotite- and higher-grade assemblages cooled following the Acadian high and returned to the surface during exhumation with minimal recrystallization. Late Devonian ⁴⁰Ar/³⁹Ar ages of white micas in these rocks support this interpretation. This contrasts with rocks from within and proximal to the EDSZ, where rocks display a new foliation defined by muscovite + chlorite reflecting crystallization under greenschist facies conditions. These greenschist-facies folia form anastomozing networks of 1-3-cm-wide discrete zones of high strain that cut microlithons containing variably digested higher-grade assemblages. Reactions producing this muscovite + chlorite assemblage consumed biotite ± garnet and other aluminosilicates in the eastern pelitic rocks and orthoclase and biotite in the western Pumpkin Ground orthogneiss. This interpretation of retrogressive reaction is confirmed by climbing ⁴⁰Ar/³⁹Ar age spectra produced from white micas within such high-strain zones. Apparent ⁴⁰Ar/³⁹Ar ages as young as 260 Ma are found within the core of the EDSZ and document that this late fabric development was induced by Alleghanian reactivation.

In both protoliths, retrogression constitutes reaction and textural softening. In the schists, the syntectonic reactions dissolved strong garnets and precipitated weaker muscovite into folia with strong preferred orientation and high contiguity. In the Pumpkin Ground orthogneiss strong, randomly-oriented feldspars dissolved and similar muscovite-rich folia developed to produce mylonitic schists. Some rocks within the EDSZ, mapped separately as Trap Falls Fm, may have been derived from bands of partially-regressed orthogneiss with especially penetrative mylonitic textures. We conclude that deformation in the EDSZ allowed the intrusion of aqueous fluids that initiated the retrograde reactions. Subsequent deformation provided activation energy and allowed continued fluid influx to drive thorough recrystallization of the higher grade assemblages, while reaction and textural softening localized strain ever more narrowly in the EDSZ.