PRESSURE-TEMPERATURE-TIME PATHS FOR THE SMITH RIVER ALLOCHTHON AND LYNCHBURG GROUP, VIRGINIA APPALACHIANS: IMPLICATIONS FOR LATE ORDOVICIAN RETROARC UNDERTHRUSTING

Rocks of the Smith River Allochthon (SRA) and Lynchburg Group (LG) are remnants of the Late Neoproterozoic eastern Laurentian extensional margin. They crop out in the Virginia Appalachians and lie in thrust fault contact, with the SRA overlying the LG. In order to decipher the tectonic evolution of the SRA and LG during Appalachian orogenesis, we have constrained P-T-t paths for amphibolite-facies metapelites from both terranes via phase equilibria modeling, geothermobarometry, and U-Pb TIMS monazite and ⁴⁰Ar/³⁹Ar muscovite geochronology.

The SRA metapelites exhibit a peak assemblage containing garnet+sillimanite+staurolite+biotite. The peak assemblage in the LG is garnet+biotite. Garnet growth P-T paths constructed from phase equilibria modeling and geothermobarometry indicate that the SRA experienced high T / low P conditions (580 ˚C, 4kb) while the LG experienced a Barrovian-style P-T path (532˚C, 6 kb to 629˚C, 9 kb). U-Pb TIMS monazite geochronology yielded ages of 443.81 +/- 0.84 Ma and 442.45 +/- 1.0 Ma for the SRA and LG, respectively. Single crystal total fusion muscovite ⁴⁰Ar/³⁹Ar ages in the SRA begin with ages of about 345-350 Ma, but yield probability distributions that lack a simple, single mode and range up to about 390 Ma. Muscovite age data from the LG exhibit a single probability distribution peak at ca. 337 Ma.

Using the data presented above combined with published data constraints for Late Ordovician calc-alkaline plutonism in the SRA and in a continental magmatic arc located east of the SRA and LG, we interpret the following tectonic evolution. By ca. 443 Ma, the LG had been underthrust beneath the SRA to a depth of ~ 34 km and experienced Barrovian metamorphism during an episode of retroarc underthrusting. Retroarc underthrusting contributed to the generation of Late Ordovician calc-alkaline plutonism in both the SRA and continental magmatic arc and subsequent high T / low P metamorphism in the SRA. This interpretation is consistent with a latest Ordovician unconformity in the foreland and in general, geodynamic models for continental arc magmatism. At ca. 337 Ma, both the SRA and LG were thrust to the northwest via the Blue Ridge fault during the initial phase of the Alleghanian orogeny, resulting in their uplift to shallow crustal levels and cooling below the muscovite closure temperature of 350 °C.