THE AGE OF MONAZITE AT THE GARNET ISOGRAD IN THE CENTRAL APPALACHIAN PIEDMONT OF PENNSYLVANIA: FURTHER EVIDENCE IN SUPPORT OF AN INVERTED METAMORPHIC GRADIENT BELOW THE EMBREEVILLE THRUST

The Embreeville Thrust, the lowest in a series of ductile, thrust-sense shear zones in high grade metamorphic rock of the central Appalachian Piedmont, places the amphibolite facies Doe Run Schist (DRS) above mainly greenschist facies Peters Creek Schist (PCS). The presence of an inverted metamorphic gradient beneath the Embreeville Thrust has been inferred based on the occurrence of the garnet-in isograd within foot wall rock of the PCS approximately 300m northwest of the fault trace or, based on simple geometric relationships, about the same distance below the fault. New U-Th-total Pb EPMA ages from monazite in PCS at the garnet isograd range from 364 ±8.0 (2σ) to 396 ±6.0 Ma, with the majority of analyzed domains yielding ages between 376 ±14.0 and 381 ±10.0 Ma. No monazite has been found in PCS samples at metamorphic grade below the formation of garnet. We interpret these monazite ages to constrain the timing of garnet growth. Thus, garnet-grade metamorphism in the PCS of the foot wall is younger than metamorphism in the DRS of the hanging wall, a necessary condition in support of an inverted metamorphic gradient. In the DRS, previous work showed that the age of monazite inclusions in garnet and staurolite range from 394 ±8.6 to 409 ±5.2 Ma, with monazite from a shear zone microlithon yielding a somewhat older age of 424 ±10.6 Ma. An arguably syn-kinematic monazite grain in the DRS is younger at 387 ±6 Ma.

Previously estimated metamorphic conditions using garnet isopleth thermobarometry, ~500 C at 600 MPa in foot wall rock (PCS) and >600 C at 700 MPa in the hanging wall (DRS), are consistent with an inverted metamorphic gradient. Preliminary results from QUIG barometry give higher pressures than garnet isopleths, but similarly indicate that hanging wall rock experienced greater depth than foot wall rock. Syn- to post-kinematic garnet with prograde growth zoning is present in the foot wall while pre-kinematic garnet, with retro-grade, Mn-enriched rims, occurs in the hanging wall. Thus, shear zone geometry, metamorphic grade, garnet zoning and fabric relationships, and the timing of metamorphism support the hypothesis that emplacement of the Doe Run Schist along the Embreeville Thrust drove prograde metamorphism in the structurally lower Peters Creek Schist.