STRAIN LOCALIZATION BY REACTION AND TEXTURAL SOFTENING IN A RETROGRADE ALLEGHANIAN DUCTILE SHEAR ZONE, SOUTHERN CONNECTICUT

We have identified grain boundary sliding and dissolution-precipitation (D-P) creep as the operative deformation mechanisms in phyllonitic East Derby shear zone, southern Connecticut. The D-P process replaced the high-grade Acadian protolith assemblage of the pelitic Wepawaug Schist including staurolite, garnet, biotite and muscovite with > 30% paragonite (pg) by chlorite and muscovite with as little as 5% pg. The replacement occurred during retrogression from amphibolite to lower greenschist facies conditions that were monitored by the gradual decrease in the pg content of the newly precipitating muscovite from 30 through 5%. Truncation of pg-rich muscovite flakes in earlier folia by pg-poor muscovite flakes in cross cutting folia provide further evidence for D-P creep. Boudinaged and broken flakes of pg-rich muscovite provide evidence against dislocation creep in the {00l} planes, whereas overgrowths of progressively pg-poorer muscovite in beards and boudin necks confirm the precipitation of retrograde muscovite during progressive deformation. Disharmonic folding of some muscovite-rich folia bounded by smooth surfaces against undeformed folia and quartz veins demonstrate significant grain boundary sliding. The cores of some muscovite fold hinges are filled with chlorite, confirming that the D-P process also involves chlorite. Pseudosection modeling of the variation of the pg content of muscovite with temperature and pressure suggests that muscovite in new cross cutting folia crystallized between 500 and >300ºC during many stages of retrogression. 40Ar/39Ar dating of unrecrystallized muscovite flakes shows that muscovite in schist passed through closure at ~365 Ma, at the end of the Acadian orogeny. In contrast, hand-picked muscovite flakes from cross-cutting folia show ages of ~305, ~290. ~280 and ~270 Ma suggesting that the recrystallization of muscovite persisted throughout the Alleghanian orogeny.