ARGON THERMOCHRONOLOGY IN THE CENTRAL VIRGINIA BLUE RIDGE AND PIEDMONT

Although the geology of the Virginia Blue Ridge and Piedmont is well studied, the timing of ductile deformation and metamorphism in the region is poorly understood. We utilize ⁴⁰Ar/³⁹Ar geochronology to resolve the temporal history of deformation and metamorphism in the Blue Ridge, the Western Piedmont, and the Chopawamsic terranes of central Virginia.

In the western Blue Ridge, the Mesoproterozoic granitoid basement complex cooled through the amphibole blocking temperature (~500˚ C) between 915 and 935 Ma, likely due to post-Grenville uplift and exhumation. Penetrative deformation in Neoproterozoic to early Cambrian cover rocks from the western Blue Ridge occurred at temperatures of ~350°C, and white mica growth/cooling ages range from 345 to 320 Ma. In the eastern Blue Ridge, amphibole ages from the basement complex range from 400 to 370 Ma. White mica cooling ages in both the basement complex and Neoproterozoic cover sequence range from 345 to 320 Ma. Penetrative deformation and metamorphism in the eastern Blue Ridge reached considerably higher temperatures than in the western Blue Ridge but both cooled through ~350°C by 320 Ma. These data suggest that Blue Ridge high-strain zones, penetrative foliations, and folds developed during the Mississippian, prior to the classic Alleghanian orogeny in the Pennsylvanian to Permian, and could be related to the Acadian orogeny. ⁴⁰Ar/³⁹Ar geochronology provides no evidence of Ordovician/Silurian tectonic events in the Blue Ridge.

White mica ages from greeschist facies metasedimentary rocks in the Western Piedmont terrane, directly east of the Blue Ridge, range from 440 to 410 Ma. In contrast, both amphibole and white mica cooling ages from metaplutonic, metasedimentary, and metavolcanic rocks in the Chopawamsic terrane range from 300 to 290 Ma. Deformation and metamorphism recorded in the Western Piedmont occurred in the early Devonian, possibly during the Taconic orogeny. Deformation and metamorphism in the Chopawamsic terrane occurred during the Alleghanian orogeny and the terrane experienced rapid uplift and cooling. These data reveal that adjacent terranes in central Virginia experienced significantly different thermal histories throughout the Paleozoic.