ELK HILL VOLCANIC COMPLEX, AN ARC FRAGMENT IN THE CENTRAL VIRGINIA PIEDMONT

Exposures of amphibolite, biotite gneiss, and pegmatite along the James River in Goochland County, VA were mapped as “Elk Hill Complex” by Taber (1913). More recent workers (e.g., Spears and Bailey, 2002; Spears et al., 2013) recognized the volcanic nature of the Elk Hill and extended it southwestward to central Cumberland County and northeastward to eastern Louisa County. Although metamorphosed to amphibolite facies, some primary volcanic features such as epidote-filed amygdules are locally recognizable. We propose that textural and geochemical evidence warrants recognition as the Elk Hill Volcanic Complex (EHVC). The EHVC occupies a structurally distinct block bounded on the northwest by the Lakeside fault and on the southeast by the Spotsylvania high-strain zone, both of which are Paleozoic transpressional mylonite zones bearing a Mesozoic brittle extensional overprint. Although lithologically similar to the adjacent volcanic Chopawamsic Formation, the EHVC is geochemically distinct. Limited sampling to date indicates that the protolith of some EHVC amphibolite is N-ocean floor tholeiitic basalt (N-OFB). Hornblende diorites of apparent arc affinity are also present. Felsite from the EHVC is a low-plagioclase rhyolite of arc affinity having a larger CN Eu (-) anomaly than we have observed in the Chopawamsic Formation. The observed range of compositions effectively rules out a rifting origin for the EHVC. The extremely low Zr content of the felsite (28 ppm) is inconsistent with typical Laurentian rocks. A TIMS U-Pb age determination and Nd isotope analyses are needed to determine the affinity of the Elk Hill Volcanic Complex.