40AR/39AR AGES AND RECRYSTALLIZATION PROCESSES IN THE ALLEGHANIAN EAST DERBY SHEAR ZONE

The timing of retrograde deformation in the East Derby shear zone (EDSZ) in south-central Connecticut is constrained to be Alleghanian by ⁴⁰Ar/³⁹Ar dating of synkinematic muscovite. The 030^otrending ductile EDSZ extends from Bridgeport 45 km northeast to Prospect, where it is cut by the western border fault of the Hartford Basin. The shear zone is characterized by phyllonites with the mineral assemblage muscovite + chlorite + albite + quartz + ilmenite and is bound by kyanite and staurolite grade pelitic schists to the east and west. The gradational boundary between peak and retrograde assemblages is marked by the disappearance of high-grade minerals, and the transposition of Acadian folia by phyllonites that dip to the northwest near Prospect and to the southeast near Bridgeport.

Electron (BSE) petrography reveals reaction textures in which prograde biotite + garnet are replaced by lower greenschist facies muscovite + chlorite + ilmenite until few to no traces of the former remain. In mm-scale domains, muscovite truncates chlorite such that P-domains contain nearly pure muscovite. These relationships can be modeled through the application of aqueous activity diagrams. At quartz-saturated greenschist facies conditions on a a(K⁺) / a(H⁺) vs a(Fe⁺²) / a(H⁺)² activity diagram, the muscovite stability field expands with decreasing temperature, displacing the muscovite-chlorite boundary, and resulting in chlorite dissolution and the precipitation of muscovite, thus explaining the muscovite-rich P-domains, whereas decreases in pressure expand the chlorite field, explaining the crystallization of chlorite in pressure shadows. Thus the distribution of minerals in the evolving fabric can be explained by incremental reactions between the minerals and the local fluid as some minerals dissolve and others precipitate to maintain equilibrium under changing P-T-X conditions.

The ⁴⁰Ar/³⁹Ar data from muscovite in the EDSZ record near plateau ages of ~360 Ma in the north and 305 and 270 Ma in the south. These isotopic data are interpreted as crystallization ages due to the petrographic evidence for chlorite-grade recrystallization. These results show that Alleghanian transpression documented in eastern New England, also affects the Acadian metamorphic high in western New England.