Acadian-age Inverted Metamorphism Below the Embreeville Thrust in the Central Appalachian Piedmont of Pennsylvania

Howell Bosbyshell, West Chester University

Abstract

The Embreeville Thrust, the lowest in a series of ductile, thrust-sense shear zones in 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 NE-SE trending fault trace or, based on simple geometric relationships, about the same distance below the fault. 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. Depletion of HREEs in this monazite is consistent with synchronous monazite and garnet growth. Furthermore, no monazite has been found in PCS samples at metamorphic grade below the formation of garnet. Thus, we interpret these monazite ages to constrain the timing of garnet growth. 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, 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.

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. 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.
Emplacement of the Doe Run Schist and metamorphism in the Peters Creek Schist marks the culmination of the Acadian orogeny in the Central Appalachians and the end of a period of metamorphism and plate convergence spanning more than 50 Ma.

Session

Session 1: Fault Zones from Top to Bottom