CORRELATING TERRANES AND THERMOTECTONIC HISTORIES OF CENTRAL VIRGINIA AND NORTHERN VIRGINIA AND MARYLAND, USA

New ⁴⁰Ar/³⁹Ar age spectrum data have been acquired from a traverse across the eastern Blue Ridge (BR) province and the Potomac Composite terrane (PCT) and western Chopawamsic terrane (CT) of the Piedmont province in central VA. Amphibole from the BR yields a plateau age of 932 Ma, recording cooling through argon closure (~500ºC) following Grenvillian metamorphism. A BR detrital white mica yields an age minimum of 932 Ma, demonstrating that most of the BR has not exceeded the argon closure temperature of white mica (~350ºC) since the Grenville orogeny. Locally, however, amphibole from mylonitic granite yields 375-390 Ma ages, and Paleozoic low-grade fabrics are defined by white mica with Devonian and Carboniferous growth ages.

Amphibole and white mica from the western part of the PCT southeast of the Mountain Run fault zone (MRFZ) yield plateau ages of 451 and 445 Ma, respectively, consistent with cooling from the Taconic orogeny. White mica age spectra from the MRFZ are complex and may suggest fault movement in both middle and late Paleozoic. Samples from the eastern portion of the PCT also yield complex results and suggest partial overprinting by Alleghanian metamorphism. Amphibole samples from CT yield plateau ages of 312-297 Ma and are interpreted to represent cooling ages from peak Alleghanian metamorphism. White mica from CT yield plateau ages of 290-284 Ma, consistent with amphibole results.

A comparison of our new data with published and unpublished data from the BR and Piedmont provinces in northern Virginia and Maryland shows both similarities and differences. In the northern BR amphibole also cooled through argon closure between 1000-920 Ma, and white mica samples also yield Devonian and Carboniferous growth ages. Correlation of Piedmont terranes and ⁴⁰Ar/³⁹Ar ages between central VA and northern VA/MD is problematic because of the intervening early Mesozoic Culpeper basin. Rocks in the westernmost PCT in MD have a Devonian white mica fabric that does not occur in PCT rocks in central VA, and Devonian white mica cooling ages are again found east of Great Falls, VA. Taconian cooling ages occur around Great Falls, similar to that found in the western PCT of central VA, and both regions appear to have been partially overprinted in the Alleghanian.