METAMORPHIC AND 40AR/39AR MUSCOVITE THERMOCHRONOLOGY STUDIES IN THE SMITH RIVER ALLOCHTHON AND LYNCHBURG GROUP, SOUTHERN APPALACHIANS: IMPLICATIONS FOR PALEOZOIC EVOLUTION OF THE EASTERN LAURENTIAN MARGIN

The Smith River Allochthon (SRA) and Lynchburg Group (LG) are part of the western Piedmont zone and Laurentian realm, respectively, in the southern Appalachians. They outcrop along the Virginia Promontory and lie in thrust fault contact, termed the Ridgeway fault, with the SRA thrust northwestward upon the LG. The juxtaposition of the western Piedmont zone and Laurentian realm is an orogen-scale boundary in the southern Appalachians that marks the closure of a seaway that had opened between the Laurentian mainland and rifted Laurentian continental fragments during the Neoproterozoic-Early Cambrian breakup of Rodinia. However, the nature and timing of juxtaposition of the SRA and LG is poorly constrained.

We have begun to construct P-T-t paths for the SRA and LG to compare and contrast their tectonothermal evolution, which will shed light on the nature and timing of their juxtaposition. Here, we present the results of our metamorphic and ⁴⁰Ar/³⁹Ar muscovite thermochronology studies. Focusing on metapelites, the SRA exhibits a peak assemblage containing garnet+sillimanite+staurolite+biotite. In the vicinity of the Ridgeway fault, this assemblage is overprinted by sericite, sericite+chloritoid, and chlorite pseudomorphs. Metapelites in the upper and lower LG contain sericite+chlorite+chloritoid+epidote and garnet+biotite, respectively.

Garnet growth P-T paths indicate that the SRA experienced low P/high T conditions (580 ˚C, 4kb) while the lower LG experienced a prograde Barrovian-style P-T path (532˚C, 6 kb to 629˚C, 9 kb). 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 both the upper and lower LG are more consistent, exhibiting a single probability distribution peak at ca. 337 Ma.

Consideration of the ca. 445 Ma plutonic complex observed in the SRA and the results of this study, the SRA is interpreted to a have a pre-Carboniferous, likely Taconic, metamorphic history and was locally remetamorphosed along with the underlying Lynchburg Group at ~ 335 Ma during thrust emplacement of the SRA. This age for juxtaposition of the SRA and LG is similar to the timing of early Alleghanian thrusting in other areas of the southern Appalachians.