DURATION OF REGIONAL KYANITE-STAUROLITE GRADE METAMORPHISM DURING THE ACADIAN OROGENY: PRELIMINARY RESULTS OF DIFFUSION MODELING OF GARNET ZONING IN THE WISSAHICKON SCHIST, SE PENNSYLVANIA

We estimate the duration of maximum temperatures in amphibolite grade rocks of the Wissahickon Schist in the Central Appalachian Piedmont by modelling diffusive modification of an inferred step function in core/overgrowth compositional zoning in garnet. Published Arrhenius parameters describing major element diffusion in Grt are not consistent, Ca and Mn diffusivities can vary by more than an order of magnitude. We compare results obtained from the parameters determined numerically by Carlson (2006) with those based on a statistical analysis of published experimental results by Chu and Ague (2015).

In the Wisshickon Schist, Silurian-aged, low-pressure, And-Sil facies series assemblages (M1) are overprinted by Devonian-aged (Acadian), moderate-pressure, Ky- and St-bearing assemblages (M2). M1 assemblages are best developed near Silurian intrusions while the M2 overprint is regional in extent. Peak metamorphism during M2 is estimated at ~600 C and 700 MPa. X-ray maps and profiles of Grt from two locations, B-22 and Ge-06-33, ~20 km apart, reveal similar Ca zoning patterns: a low-Ca core surrounded by a high-Ca overgrowth, with a diffusively modified boundary of ~50 μm. Mn zoning differs between samples. In Ge-06-33, a relatively unzoned core gives way to bell-shaped growth zoning, a profile not amenable to diffusion modeling. In B-22, growth zoning preserved within a 500 µm core is surrounded by a homogeneous, lower Mn overgrowth. This pattern enables identification of a diffusively modified core/overgrowth boundary ~150 μm wide.

The duration of max T (600 C) required to reproduce Ca zoning is the same in both samples: ~16 Ma using Chu & Ague’s (2015) parameters vs. ~2 Ma using Carlson’s (2006). The time required to reproduce Mn zoning is <1 Ma and ~2 Ma, respectively. The preservation of growth-zoning in Mn at the sub-500 μm scale is consistent with the short duration of max T obtained from both sets of Mn and Carlson’s (2015) Ca parameters. This short duration regional metamorphism requires a transient heat source that is, at present, unidentified.