GEOCHEMISTRY OF THE ARVONIA FORMATION, CHOPAWAMSIC TERRANE, VIRGINIA: IMPLICATIONS FOR SOURCE AREA WEATHERING AND PROVENANCE

The Arvonia Formation in the Piedmont of central Virginia ranges primarily from slates to garnet schists, which unconformably overly Middle Ordovician metavolcanic rocks. It has typically been interpreted as Late Ordovician based on poorly preserved fossils, perhaps having formed as a successor basin following accretion of the Chopawamsic arc terrane to Laurentia. If true, the Arvonia sedimentary basin could have received substantial detritus from Laurentian sources. However, detrital zircon results reported by Bailey et al. (2008 GSA abstract) from basal quartzites in both the Arvonia and correlative Quantico Formation show a paucity of 1- 2 Ga ages, a strong peak at 440 - 470 Ma, and a youngest age of 390 Ma. These results suggest: 1) that the Arvonia may be as young as Devonian; and 2) minimal to no contribution from Laurentian sources. Whole-rock compositions of shales can provide valuable constraints on source area characteristics, thus we used this approach to evaluate further the origin of the Arvonia Formation. Twelve samples collected along the length of the outcrop belt show similar major element compositions, which are typical of shales. Chemical Index of Alteration values (67-78) indicate an intermediate level of source weathering. A majority of the samples show uniform rare-earth element (REE) patterns that are light REE-enriched (La_N/Lu_N = 8-11), with negative Eu-anomalies (Eu/Eu* = 0.63-0.78). Values of Th/Sc, a monitor of mafic vs. felsic sources and the average provenance, show little variation (0.64-0.79). A variety of additional trace element values or ratios indicate that Arvonia sediments are similar to shale composites (thus upper continental crust). Nd-isotopic compositions for three samples are similar, with ε_Nd = -7.5 to -8.6 (at 400 Ma), and T_DM ranging from 1.7 to 1.9 Ga. Collectively, these results are consistent with derivation of Arvonia sediments from a single source or well-mixed sources of typical upper crustal composition. The isotopic data are compatible with a Laurentian provenance, however, they are equally compatible with a strictly Chopawamsic crustal source based on available Nd isotopic data. The latter interpretation provides a plausible way to reconcile our results with the detrital zircon ages, but requires that Arvonia sediments were deposited prior to accretion.