TRANSITION FROM THE LYNCHBURG GROUP TO THE ASHE AND ALLIGATOR BACK METAMORPHIC SUITES: NEW DATA AND NEW INTERPRETATIONS FROM DETAILED MAPPING ALONG THE BLUE RIDGE PARKWAY IN SOUTHERN VIRGINIA

Decades of reconnaissance mapping have depicted rocks of the Lynchburg Group (LG) in the eastern Blue Ridge (eBR) of central VA transitioning into the Ashe and overlying Alligator Back metamorphic suites (AMS and ABMS) of NW NC. But are these major rock units stratigraphically equivalent, with changes in lithologic facies and increasing metamorphic grade along strike to the SW, or are they distinct sedimentary packages, separated by regional unconformities and(or) juxtaposed by faults? Detailed geologic mapping in LG, AMS, and ABMS rocks along the Blue Ridge Parkway in southern VA provides significant new data to address these questions.

LG and AMS clastic rocks in this area consist of a lower metagraywacke unit and an upper unit dominated by muscovite-quartz metasandstone and graphitic schist; these lithologies are interlayered with amphibolite and altered ultramafic rocks. The rocks contain multiple foliations and are complexly folded. Metamorphic grade increases to the southeast, from garnet-grade at the Mesoproterozoic basement contact (Red Valley fault zone) to kyanite-grade at the ABMS contact. Coarse-grained and thick-layered metagraywacke in the lower unit is lithologically similar to AMS mica gneisses farther south in the eBR of NC. The stratigraphic sequence (lower metagraywacke and upper metasandstone and graphitic schist) is identical to a recently mapped lower greenschist-facies LG package north of Lynchburg in the central VA eBR.

Distinctively thin-layered ABMS clastic rocks contain more muscovite and much less graphite than underlying LG and AMS rocks, and are interlayered with greenstone, amphibolite and some altered ultramafic rocks. ABMS rocks are highly deformed and were tectonically emplaced over LG and AMS rocks: ABMS muscovite-phyllite at the contact are L-S tectonites, and units and folds are truncated along the contact.

Preliminary data support these conclusions: 1) AMS metagraywackes, mica schists, and amphibolite comprise the lowest sequence in the eBR; 2) LG metasandstone, metagraywacke, graphitic schist, amphibolite and altered ultramafic rocks stratigraphically overly the AMS; and 3) ABMS rocks were transpressionally thrust onto the AMS and LG sequence in VA.