Geochemistry of the Arvonia Formation, Chopawamsic terrane, Virginia: Implications for source area weathering and provenance

Abstract

The metasedimentary Arvonia Formation in the Piedmont Province of central Virginia ranges primarily from slates to garnet schists, which unconformably overly Middle Ordovician metavolcanic rocks of the Chopawamsic arc terrane. The formation 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 to Laurentia. However, alternative interpretations have been proposed, including deposition outboard of Laurentia on Chopawamsic arc crust (that is, prior to accretion). Furthermore, the origin of the Chopawamsic arc as peri-Laurentian or peri-Gondwanan is an unresolved issue. We report whole-rock major and trace element compositions as well as Nd-isotopic compositions of the Arvonia Formation, and use these results to evaluate further its origin. Samples collected along the length of the outcrop belt show similar major element compositions, which are typical of shales. Chemical Index of Alteration values (68-89), corrected for K-metasomatism, indicate intermediate to extreme levels of source weathering. A majority of the samples show uniform rare-earth element (REE) patterns that are light REE-enriched with negative Eu-anomalies. A few samples display anomalous REE patterns, indicative of some disturbance of the light REE at approximately the time of deposition. A variety of trace element values or ratios indicate that Arvonia sediments are similar to shale composites (thus upper continental crust). Nd-isotopic compositions for eight samples are similar, with ε_Nd = −5.6 to −8.9 (400 Ma). 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.