Limited, localized nonvolatile element flux and volume change in Appalachian slates

Abstract

Geometric evidence for major nonvolatile volume losses at several Appalachian slate localities contradicts geochemical evidence for minimal compositional changes during low-grade metamorphism of shales. Relative to adjoining microlithons, spaced cleavage zones are depleted in Si (quartz), Na (albite), and, for carbonate-rich slates, Ca, Mn, and Mg (calcite and dolomite). Zr, Ti, Al, P, and Fe are enriched in cleavage zones; the greatest enrichments are in Zr. Proportional K gains and Na and Si losses suggest that cleavage fractionation combines stoichiometric reactions (e.g., K + + albite + chlorite = muscovite + SiO 2 + Na + ) with pressure solution of quartz and carbonate minerals. Cleavage and sorting (pelite to psammite) fractionation patterns are similar but can be distinguished by higher Zr in Al-rich cleavage zones relative to Zr in Al-rich pelitic rocks. The compositional similarities of homogeneous shales and slates do not indicate large volume losses during slate metamorphism. For Paleozoic slates from localities where large volume losses have been hypothesized on geometric grounds (Martinsburg slate belt of Pennsylvania and New Jersey, Meguma Group of Nova Scotia, and Taconic slate belt of Vermont and New York), slate compositions show no evidence of large nonvolatile volume losses relative to likely shale protoliths. Detailed sampling of slates folded at hand specimen to quarry scales indicate that substantial element and volume fluxes are limited to the submeter scale. In small-scale folds, volume losses in tight inner arcs appear to be balanced by volume gains in adjoining areas. The complications of metamorphic recrystallization and heterogeneous strain are probably responsible for erroneous interpretations of large nonvolatile volume losses in these Appalachian slate localities.