Allochemical retrograde metamorphism in shear zones: an example in metapelites, Virginia, USA

Abstract Metapelites in the Altavista area, southwest Virginia Piedmont, USA, underwent allochemical hydrothermal retrograde metamorphism in synmetamorphic shear zones. The metapelites of the Evington Group were metamorphosed in a prograde sequence of chlorite, staurolite, and sillimanite zones. Garnet–biotite geothermometry and phase relations support eastward increasing metamorphic grade, ranging from 570° C in the staurolite zone to 650° C in the sillimanite zone at c. 5.8 kbar. Sillimanite‐zone rocks later underwent progressive retrogression around shear zones which acted as fluid conduits. Retrograde assemblages are successively zoned around the shear zones with staurolite‐, chloritoid‐ and kyanite‐bearing assemblages. The shear zones commonly contain kyanite or tourmaline veins. Applicable phase equilibria indicate that retrogression occurred during isobaric cooling through c. 200–270° C. Rock compositional changes with retrogression occurred in steps: SiO₂ was gained in the early stages of the retrogression but lost in the late stages; Al₂O₃, K₂O, and H₂O were increasingly gained through the sequence; CaO was increasingly lost. Addition of H₂O and decreasing temperatures resulted in new ferromagnesian minerals (staurolite, chloritoid, chlorite) and changes in H₂O, SiO₂, Al₂O₃, K₂O, and CaO contents produced muscovite and sodic plagioclase. Subsequent to prograde metamorphism, deeply derived fluids migrated upwards along shear zones, providing fluid and energy for the retrograde reactions. The sheared rocks underwent fluid infiltration with fluid fluxes of 1.8 × 10⁷–4.3 × 10⁷ cm³/cm² corresponding to minimum estimated fluid‐to‐rock ratios of 7.5–21 as a function of position within the shear zone. Fluid flow was from high to low temperature early and low to high temperature later in the retrogression.