DEVONIAN DECOMPRESSION IN HIGH-GRADE METAPELITE FROM THE CENTRAL APPALACHIAN PIEDMONT: RESULTS FROM DIFFUSION MODELING OF CA IN GARNET AND MONAZITE GEOCHRONOLOGY

Textural observations, equilibrium assemblage modeling and garnet isopleth thermobarometry document decompression of the Sycamore Mills Formation, pelitic gneiss which occurs in the Avondale nappe along the Rosemont shear zone in southeastern Pa. Large garnet is characterized by cores containing abundant crystallographically oriented rutile needles up to 200 µm long. Theriak-Domino models suggest that the mineral assemblage within garnet cores, Pl + Kfs + Ky + Rt + Qtz + Ms ± Bt (+Liq), is stable between 750 – 800°C at pressures greater than 1.0 GPa. Results from Zr-in-rutile thermometry are consistent with these temperature estimates. Quartz inclusions are separated from garnet by a narrow (50 – 150 µm) intergrowth of plagioclase and Al2SiO5. X-ray composition maps show pronounced Ca-depletion halos in garnet adjacent to these intergrowths. These textures suggest that plagioclase formed from garnet breakdown and that Ca-depletion in garnet is the result of diffusion during decompression. We use a simple one component radial diffusion model to estimate the cooling rate and time required to replicate the measured Ca profile in the depletion halos in garnet. Mass balance considerations support the efficacy of this approach. With a starting temperature estimate of 800°C, the best fit was achieved with a cooling rate of 60°C/Ma in 3 million years. Thermobarometry suggests that this corresponds to decompression of 0.5 GPa.

X-ray composition maps of monazite show distinct zoning in Y content. Low-Y cores are surrounded by high-Y rims in nearly all matrix grains that were mapped. Monazite inclusions in garnet lack the high-Y rims. High-Y rims likely formed during garnet breakdown to plagioclase. U-Th-total Pb ages from more than 20 compositional domains in multiple samples record low-Y monazite growth beginning at ~440 Ma and continuing throughout the Silurian. A sharp increase in Y content is observed in monazite at 410 Ma, which likely corresponds to the time of rapid high-T decompression. Decompression here has been interpreted to result from rapid uplift in a transtensional regime following a change from sinistral to dextral kinematics in the middle Devonian. The new monazite results suggest that uplift occurred earlier, possibly reflecting extrusion of deeply buried rocks during older sinistral transpression.