METAMORPHIC EVOLUTION OF THE WISSAHICKON SCHIST: A MONAZITE PETROCHRONOLOGICAL INVESTIGATION

Central to understanding the relative importance of advective and conductive heat transfer mechanisms in regions of thickened continental crust is the generation of precise and accurate pressure-temperature-time paths. Here, we present a petrochronological investigation of a suite of metapelites from the Wissahickon Formation of the Central Appalachian Piedmont—an exceptionally well-preserved record of progressive metamorphism of mudrock protoliths. Garnet-bearing pelitic-to-psammitic samples were collected from the turbidite sequence exposed along the Wissahickon Valley Park, Philadelphia, USA. E-beam (SEM, EPMA) textural and chemical analysis of major and accessory phases was used to link monazite growth to the Barrovian-type metamorphic paragenesis. Phase equilibria modeling combined with conventional thermobarometry (GASP barometry) yield PT constraints of 6.5-7.5 kbar and 600-650ºC. Results from monazite laser ablation split stream (LASS) analysis shows that monazite trace element compositions vary systematically with metamorphic grade as seen in Gd/Yb vs. 206-238 Age diagrams. This is consistent with garnet imposing a predominant control on monazite chemistry. Pseudosection-derived isomodes indicate that increasing modal abundances of garnet toward higher metamorphic grade primarily reflects an increase in pressure during regional metamorphism. Monazite U-Pb dates from sample ASWH13 define a coherent Acadian population, between ~380 and ~370 Ma, whereas dates from samples ASWH10 and ASWH13 are consistent with monazite growth during prograde and retrograde portions of the PT path. These results provide further constraints on the metamorphic evolution of Acadian orogenesis.