TRANSTENSIONAL EXHUMATION OF HIGH-TEMPERATURE HIGH-PRESSURE METAMORPHIC ROCK: AN EXAMPLE FROM THE ROSEMONT SHEAR ZONE, CENTRAL APPALACHIAN PIEDMONT

Textural observations, equilibrium assemblage modeling and garnet isopleth thermobarometry document isothermal decompression of the Sycamore Mills Formation, pelitic gneiss in the Avondale nappe, along the Rosemont shear zone near Media, Pa. The rock contains the assemblage Grt + Sil + Bt + Ksp + Pl + Qtz with accessory Ilm, Rt, Zrn, Mnz, and Mag. Garnet as large as 1.5 cm diameter is characterized by cores containing abundant crystallographically oriented rutile needles up to 200 µm long. Coarse kyanite (<1 mm) and zincian staurolite (~0.3 mm long) inclusions are also present in garnet cores. Garnet rims contain relatively coarse rutile grains (0.25 – 0.5 mm), which parallel a fabric defined by the long dimension of quartz inclusions. Many quartz inclusions are separated from garnet by a narrow (50 – 150 µm) symplectic intergrowth of plagioclase and Al₂SiO₅. X-ray composition maps show pronounced Ca depletion halos in garnet adjacent to plagioclase.

Theriak-Domino models suggest that the mineral assemblage within garnet cores, Fsp + Bt + Ky + Rt + Qtz, is stable between 750 – 800 °C over a pressure range of 0.9 to 1.2 GPa. Results from Zr-in-rutile thermometry are consistent with these temperature estimates. While diffusion is likely to have modified the original composition of garnet cores, measured grossular content matches the modeled composition at these conditions. Plagioclase-Al₂SiO₅ intergrowths on quartz inclusions and Ca-depletion halos in surrounding garnet are the result of near isothermal decompression. Whitney (1991) proposed that such textures form from reactions involving fluid, introduced along fractures in garnet, with aqueous Ca⁺²as a product. Isopleth thermobarometry using the composition of garnet from Ca-depletion halos indicate metamorphic conditions of 700 – 750°C and 0.6 GPa.

Monazite geochronology is ongoing, however, a monazite inclusion along a fracture in garnet yielded an age of 399 ±14 Ma (Pyle et al., 2006), similar to the age of syn-tectonic monazite in the Rosemont shear zone. We propose that curvature of the orogen at the NY promontory was effectively a restraining bend during a sinistral transpressive regime in the Silurian. A change from sinistral to dextral kinematics by the middle Devonian produced a transtensional regime resulting in rapid uplift of the Sycamore Mills rock.