METAMORPHIC DIFFERENTIATION BY DISSOLUTION-PRECIPITATION CREEP IN FAULT-ROCK PHYLLONITES

Phyllonites of the East Derby Shear Zone (EDSZ) in south central Connecticut have been analyzed by optical and electron microscopy, electron-microprobe analysis (EMPA), and ⁴⁰Ar/³⁹Ar geochronology. Results reveal multiple generations of lower greenschist facies Permian cleavages overprinting a kyanite-grade Devonian schistosity in the Wepawaug Schist. The EDSZ is a ~500m wide greenschist facies shear zone and displays progressive replacement of staurolite, garnet and biotite by muscovite ± chlorite as the zone is approached. Muscovite with Na-rich (0.2-0.3 Na-cpfu) Devonian cores, and Na-poor (0.05 Na-cpfu) Permian rims, identified by wavelength dispersive element mapping and quantitative EMPA, confirm the high and low-grade crystallization of cores and overgrowths, respectively, and establishes a dissolution-precipitation mechanism for this process.

Early generations of retrograde phyllonitic cleavages have coexisting Na-poor muscovite and chlorite, whereas later cross-cutting cleavages contain exclusively Na-poor muscovite. The differentiation of muscovite from chlorite (and from quartz and albite) can be explained by changes in the relative solubilities of these phases as calculated by SUPCRTBL (Zimmer et al, 2016). During later deformation the solubilities of chlorite, quartz and albite increased relative to muscovite at sites of high normal stress. This resulted in the preservation and neocrystallization of muscovite in cross-cutting cleavage domains, while only chlorite and quartz precipitated locally in micro-extensional sites. New ⁴⁰Ar/³⁹Ar age spectra of microdrilled muscovite from high and low-grade folia and from pseudomorphic muscovite after staurolite date multiple stages of muscovite crystallization during Alleghanian orogenesis. The metamorphic differentiation observed within the phyllonites of the EDSZ is thus a tectonically driven chemical process where differential stresses caused local changes in the relative solubilities of muscovite and adjacent minerals and led to the concentration of less soluble muscovite in the youngest cleavages.