A PETROGRAPHIC AND GEOCHEMICAL EAST VERSUS WEST ANALYSIS OF THE NEOPROTEROZOIC CATOCTIN FORMATION IN THE BLUE RIDGE ANTICLINORIUM, CENTRAL VIRGINIA

The Neoproterozoic (ca. 570 Ma) Catoctin Fm. of central Virginia and Maryland provides an example of the preserved bimodal extrusive sequence (and subaerial to subaqueous clastic rocks) associated with late Proterozoic extension and rifting of the Rodinia supercontinent. This sequence consists of metamorphosed bimodal mafic volcanic rocks (metabasalt), phyllites and interbedded clastic metasedimentary rocks that support the eastern and western flanks of the Blue Ridge Anticlinorium, a breached Alleghanian-age structure exposing a Grenville basement core.

Most of the previous study of the Catoctin Fm. has concentrated on the western flank of the Blue Ridge Anticlinorium. Thus, petrographic and geochemical techniques (e.g., pXRF, ICP-MS) were used to develop a greater understanding of the stratigraphic relationships of the Catoctin metavolcanic rocks and adjacent units within the current tectonic model, as well as analyze an east-to-west traverse across the anticlinorium. Additionally, this petrographic and geochemical characterization will be applied to a group of metabasalts and phyllites (Oronoco Greenstone) that lie within the anticlinorium. This study may allow the determination of whether these rocks correspond to the Catoctin Fm. or the older (ca. 750-738 Ma) upper Lynchburg Group.

Preliminary petrographic analyses of Catoctin Fm. metabasalts indicate lower to upper greenschist metamorphism dominated by chlorite ± actinolite + epidote + plagioclase (sodic) + sphene + ilmenite ± quartz ± calcite. Plagioclase lathes (subophitic texture), with albite polysynthetic twinning, are surrounded by chlorite and epidote. Pyroxene is absent and chlorite ± actinolite is the major mafic phase. Sphene is commonly adjacent to ilmenite and appears to be secondary, possibly as a result of Ti-rich fluids. Veins (and possible amygdules) are filled with quartz, epidote or calcite, most likely a result of post-metamorphic fluids. Interbedded lavender phyllites appear to be of two varieties: a fine-grained clastic (quartz + oxides ± zircon) with a clay mineral matrix, while the other may represent metamorphosed tuffaceous material, some with Catoctin equivalent metabasalt fragments.