PETROGRAPHIC AND SEM ANALYSIS OF POSSIBLE GRANULITE UHT/UHP METAMORPHISM OF PELITIC GNEISS, AVONDALE MASSIF, SOUTHEASTERN PENNSYLVANIA

This study focuses on pelitic gneiss from the Sycamore Mills formation of the Avondale massif near Media, Pa. The rock contains the assemblage grt + sil + bt + ksp + pl + qtz with accessory ilm, rut, zrn, mnz, and mag. Garnet as large as 1.2 cm, with subhedral but well-defined 0.5 cm cores is present. EDS x-ray analysis shows that the garnet is strongly zoned in grossular component, from grs 0.5 to 0.15 from the outer to inner rim, with a sharp step to the grs 0.17 - 0.18 core. Sps concentration is greater in the core than rim. Alm and sps increase sharply in the outermost rim where pyr decreases. Garnet cores are characterized by abundant inclusions of rutile needles up to 75 µm long and 2 µm in cross-section. Ilmenite inclusions of similar size are also present, but are considerably less abundant. Rutile needles are disseminated throughout the garnet core but are also concentrated in sinuous to straight bands 75 to 150 µm wide. Within these bands, rutile needles have a preferred orientation, possibly parallel to crystallographic planes in the enclosing garnet. Coarse kyanite (<1 mm) and zincian staurolite (~0.3 mm long) inclusions are also present in the core. 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, but not all, quartz inclusions are separated from garnet by a narrow (50 – 100 µm) rim of plagioclase intergrown with Al₂SiO₅. X-ray composition maps show pronounced Ca depletion halos in garnet adjacent to plagioclase.

Preliminary Theriak-Domino models suggest minimum pressures of 0.85 GPa at 750 – 800C, although many studies attribute similar occurrences of rutile needles in garnet to open system precipitation at considerably higher P and/or T. We plan to further assess temperature using Zr in rutile thermometry. The formation of plagioclase within garnet rims is consistent with isothermal decompression. P-T estimates with TWQ, using the composition of low-Ca garnet, adjacent plagioclase rims on quartz and matrix biotite yield 0.55 GPa at 600C. The 475 – 480 Ma age of monazite inclusions in garnet from this sample (Pyle et al., 2006) is identical to the age of magmatic zircon in the nearby Wilmington Complex arc. This suggests that this metamorphism is related to arc formation.