FROM THE IAPETUS TO THE ATLANTIC: GEOLOGY, PETROLOGY, AND GEOCHRONOLOGY OF COVER ROCKS IN THE EASTERN BLUE RIDGE, CENTRAL VIRGINIA

In central Virginia, the eastern Blue Ridge includes a cover sequence of low-grade meta-igneous and meta-sedimentary rocks that record the opening of the Iapetus Ocean at the end of the Neoproterozoic. We combine new geological mapping, stratigraphic analysis, petrology, and geochronology to better understand the tectonic setting and temporal history of the eastern Blue Ridge.

The Catoctin Formation forms a thick sequence of primarily thoellitic meta-basalt, but also includes meta-arkose and phyllite near its top. Pillow lavas are common in the upper Catoctin Formation and evidence for marine effusion of lavas. The Catoctin Formation is overlain by a thick package of phyllite, meta-sandstone, meta-graywacke, tuffaceous meta-sandstone, and marble. Collectively, we correlate these strata with the Evington Group to the south and interpret the sequence to be marine. Locally, previous workers mapped both dacitic meta-volcanic rocks and quartz diorite porphyries in this sequence.

Our petrographic analysis indicates that the dacitic meta-volcanic rocks are fine-grained mica-rich meta-sandstones with quartz clasts. LA-ICP-MS U-Pb detrital zircon geochronology reveals a dominate, but broad, age population between 1.0 to 1.2 Ga with a few older grains back to 1.8 Ga. We interpret these rocks to have a Laurentian provenance.

The porphyritic rocks occur in narrow NE-striking dikes that obliquely cross-cut older meta-sedimentary units. Chemically, these rocks are phonolites (~53% SiO₂, ~15% alkalis) and contain broken phenocrysts of alkali feldspars in a hornblende/biotite-rich matrix. LA-ICP-MS U-Pb apatite dating of two samples of porphyritic phonolite yield late Jurassic ages (155 to 160 Ma). We infer these dikes to represent off-axis rift margin magmatism related to the nascent opening of the Atlantic Ocean.