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
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.