DETRITAL ZIRCON GEOCHRONOLOGY ACROSS THE CHOPAWAMSIC FAULT, WESTERN PIEDMONT OF VIRGINIA, AND ITS BEARING ON EARLY PALEOZOIC TECTONIC DEVELOPMENT IN THE SOUTHERN APPALACHIANS

The Late Ordovician-Early Silurian Chopawamsic fault bisects early Paleozoic rocks of the western Piedmont of Virginia into a probable Laurentian accretionary complex, the Potomac terrane (Pt), to the west and an Iapetan arc of unknown crustal affinity, the Chopawamsic terrane (Ct), to the east. We have analyzed more than 1000 individual detrital zircon grains from 11 samples of metasedimentary rocks from these terranes as well as two previously mapped successor basins using laser ablation ICP-MS, U-Pb geochronology. Our results reinforce the importance of the Chopawamsic fault and lend insight into the tectonic landscape prior to and after the accretion of the Ct to Laurentia.

Samples from the pre-Ordovician Pt display various peak modes from 1.00 – 1.25 Ga; almost all analyses in these samples span from c. 500 Ma to c. 1.6 Ga. These results support a Laurentian provenance for the Pt.

Metasedimentary samples from the Middle to Late Ordovician Ct are dominated by detrital zircons between 450 – 470 Ma; these ages likely represent recycled debris from adjacent volcanic rocks in the Ct. However, one sample from the Ct displays a sub-population of Proterozoic zircons ranging from 600 Ma to 1.2 Ga; this set provides the only indication of any basement input to these samples. Zircons from 600 – 800 Ma may reflect the influence of a peri-Gondwanan crustal basement to the Ct.

Samples from the Arvonia successor basin, which unconformably overlies the Ct, display dissimilar detrital inputs. A sample from the Bremo quartzite exhibits peak modes at c. 460 Ma and 1.01 Ga, while a sample from the Buffards Formation consists mostly of Middle Silurian, c. 425 Ma, detrital zircons. The age disparities between these two samples may reflect a transition in sediment provenance from underlying older terranes (potential input from both Ct and Pt) to the introduction of contemporaneous magmatism to the Arvonia depositional system. A sample from a smaller unit of metasedimentary rocks (Storck area) previously interpreted to be related to the Arvonia basin displays a wide distribution of detrital ages ranging from 500 Ma to 2.25 Ga, with principal modes at c. 525 Ma, 605 Ma, 945 Ma and 1.14 Ga; it is uncertain how or if this sample may be related to the Arvonia basin, but the distribution of detrital ages is consistent with input from a Gondwanan source.