DECIPHERING THE METAMORPHIC AND DEFORMATION HISTORY OF THE AVONDALE MASSIF USING PETROGRAPHIC AND SEM ANALYSIS

The Avondale Massif is a fault bounded body of heterogeneous gneiss which underlies a portion of the central Appalachian Piedmont of Pennsylvania. The Avondale is emplaced above the West Chester massif by the Street Road fault and is separated from lower Paleozoic volcanic arc rocks to the east by the Rosemont shear zone. This study builds upon recent work on pelitic gneiss in the Avondale massif which identified an early high temperature and/or high pressure period of metamorphism, based on the occurrence of rutile needles oriented parallel to crystallographic directions in garnet. Evidence for isothermal decompression of these rocks, based on the presence of plagioclase halos on quartz inclusions in the rims of this garnet has also been discussed. Preliminary thermobarometric modeling and results from Zr in rutile thermometry suggest that these rocks were uplifted from pressures greater than 0.85 GPa at temperatures around 800C through 0.55 GPa at 600C (Trice and Bosbyshell, 2014).

We have identified an additional sample with oriented rutile needles in garnet cores, from a location on the eastern margin of the massif along the Rosemont shear zone. Due to the proximity of the shear zone, it exhibits different fabrics and preserves different portions of the metamorphic history. Two generations of garnet may be present. Anhedral garnet, up to .5 cm in diameter, containing rutile needles is present along with relatively euhedral garnet no greater than 1 mm in diameter, which only contains inclusions of quartz. Plagioclase halos on garnet have not been identified. Intracrystalline strain is characteristic of deformation under amphibolite facies conditions. Plagioclase crystals display bent and tapered deformation twins and bulge nucleation recrystallization as well as evidence for subgrain rotation recrystallization. Quartz occurs as polygonal crystals and appears relatively strain free. This contrasts with plagioclase and quartz microstructures in samples from previous studies. Within these rocks, quartz exhibits undulose extinction with lobate grain boundaries indicative of lower grade conditions. Some plagioclase crystals display tapered twins but no evidence of recrystallization has been observed. We plan to examine additional samples to build a complete understanding of the metamorphic history.