INVERTED METAMORPHISM BELOW THE EMBREEVILLE THRUST IN THE CENTRAL APPALACHIANS: THERMOBAROMETRY AND MONAZITE GEOCHRONOLOGY

The Embreeville Thrust is the lowest in a series of ductile, thrust-sense shear zones in high grade metamorphic rock of the central Appalachian Piedmont where thrust stacking resulted in the highest grade metamorphic rocks occurring in the structurally highest sheet. The Embreeville Thrust places the amphibolite facies Doe Run Schist (Grt+St+Bt+Ms+Pl+Qtz±Ky±Chl) above mainly greenschist facies Peters Creek Schist (Grt+Ms+Chl+Bt+Pl+Qtz). The garnet-in isograd occurs in the Peters Creek Schist approximately 300m northwest of the fault trace or, given an inferred dip of ~45°, about the same distance below the fault. Garnet in Peters Creek Schist exhibits curved inclusion trails suggesting syn- to post-kinematic garnet growth. Garnet core compositional isopleths intersect at 520 C and 600 MPa, which likely indicates considerable over-stepping of the garnet-in reaction. Prograde garnet zoning suggests that maximum temperatures were somewhat higher. Peak metamorphic conditions in the Doe Run Schist in the hanging wall are estimated to have been in excess of 600 C at approximately 700 MPa based on the presence of staurolite and kyanite. Garnet in the hanging wall exhibits a sharp increase in Mn content at the rim, indicating some degree of garnet dissolution which we interpret as the result of retrograde metamorphism driven by fluids released from the footwall during thrusting.

The timing of peak metamorphism in the Doe Run Schist is constrained by monazite ages, including monazite inclusions in garnet and staurolite, which range from 394 ±8.6 (2σ) to 409 ±5.2 Ma. Monazite within relatively coarse-grained microlithons in mylonitic rock from the shear zone yields a somewhat older age, 424 ±10.6 Ma. Elongate, asymmetric, likely syntectonic monazite in the same sample is younger at 387 ±6 Ma. Monazite has not been found in the Peters Creek Schist, but xenotime may constrain the timing of metamorphism in the footwall. These observations support the hypothesis that emplacement of the Doe Run Schist drove prograde metamorphism in the structurally lower Peters Creek Schist.