THE GEOLOGY OF THE CATOCTIN FORMATION AND POSSIBLE ASSOCIATED METAGABBROS IN THE EASTERN BLUE RIDGE, CENTRAL VIRGINIA

The Catoctin volcanic province in the central Appalachians consists of primarily mafic lava flows extruded during a continental rifting event in the late Ediacaran (570-550 Ma). On the eastern limb of the Blue Ridge anticlinorium in central Virginia the Catoctin Formation overlies the metasedimentary Lynchburg Group and underlies the Cambrian Candler Formation. In southern Albemarle County the Catoctin Formation is a 1.5-2.0 km thick package of metabasalts with thin layers of metasedimentary rocks. A series of metagabbroic sills and dikes cut both the Mesoproterozoic basement complex and the Lynchburg Group, but do not intrude the Catoctin Formation. A large metagabbroic body, known as the University Amphibolite, forms a ~20 km long and up to 600 m thick sill in the lower section of the Lynchburg Group. The feeder dike for the University Amphibolite is ~1 km wide and cuts upward from the basement.

Both the Catoctin metabasalts and metagabbros experienced Paleozoic greenschist facies metamorphism. Catoctin metabasalts are well foliated with chlorite, actinolite, epidote, and magnetite. Metagabbros are poorly foliated and composed of hornblende and plagioclase­­. Major and trace element compositions of metabasalt and metagabbro samples are similar with SiO₂ ranging from 45 to 52 wt%, MgO <7.5 wt% and TiO₂ >1.8 wt%. These are typical values for metamorphosed tholeiitic basalts. Samples of both show uniform rare-earth element (REE) patterns that are moderately light-REE enriched (La_N/Yb_N = 3-9), with only slight positive or negative Eu-anomalies. The metabasalts and metagabbros display HFSE patterns suggestive of a shared origin.

Curiously, in the eastern Blue Ridge there are no dikes present which connect the metagabbroic bodies to the Catoctin Formation. Two possible scenarios for the intrusion of the metagabbros include: (1) they are the mafic component of a bimodal magmatic event that occurred during a failed rifting event earlier in the Neoproterozoic (e.g. Robertson River Igneous Suite or Crossnore Complex) or (2) they were emplaced contemporaneously with extrusion of Catoctin metabasalts, but did not serve as a magmatic conduit for surface volcanism. Obtaining a crystallization age for these metagabbros is critical for understanding the tectonic history of the eastern Blue Ridge.