THE ELLISVILLE GRANODIORITE: A STITCHING PLUTON IN THE WESTERN PIEDMONT OF VIRGINIA?

The Ellisville pluton is a tadpole-shaped body of biotite granodiorite that has been depicted as intruding across the Chopawamsic fault in the western piedmont of Virginia. The Chopawamsic fault is a regionally significant, SW-NE trending fault that separates metaclastic rocks of the Potomac terrane to the west from mainly metavolcanic rocks of the Chopawamsic terrane to the east; it may represent the main Iapetan suture in the southern Appalachians. The main body of the Ellisville pluton lies within the Potomac terrane, and has previously yielded a U-Pb zircon SHRIMP age of 444±12 Ma. Our recent fieldwork has verified the presence of narrow (~1.5 km wide), southern ‘tail’ of the pluton in the adjacent Chopawamsic terrane; this tail appears to be contiguous with the main body, suggesting that the Chopawamsic fault intersects the Ellisville granodiorite where its tail joins the main body. Both portions of the pluton consist almost entirely of medium grained biotite granodiorite; the main body is mainly massive and the tail is typically foliated. Some maps also depict the Ellisville ‘tail’ as extending SW for approximately 6 km and connecting with the c. 457 Ma Columbia pluton, a major granitoid body in the Chopawamsic terrane.

The apparent stitching relationship of the Ellisville pluton makes it a potentially ideal geochronological target for constraining the age of youngest movement along the regionally significant Chopawamsic fault. In order to confirm this apparent relationship, it is imperative to demonstrate that the Ellisville main body and tail are comagmatic. We are investigating this relationship through field observations, petrographic studies, geochemical analysis, and geochronological comparisons between the two areas. In this vein, a total of eighteen samples were collected from the Ellisville main body, tail, and Columbia pluton for petrographic and geochemical evaluation. Published USGS geochemical data from fifteen samples of the Ellisville main body are also included in the analysis. Of the eighteen new sample sites, two were chosen for U-Pb zircon TIMS analysis, one from the main body and one from the tail. Our results to date support the idea that the main body and tail constitute portions of a single pluton and that latest motion on the Chopawamsic fault is Late Ordovician or older.