Paper No. 4
Presentation Time: 8:55 AM
Detrital Zircon Geochronology of the Chopawamsic Terrane, Virginia Piedmont: Evidence for a Non-Laurentian Provenance
The early Paleozoic Chopawamsic continental arc terrane forms a distinctive lithotectonic element of the Piedmont Zone in the central Appalachians. Terranes in the Carolina Zone are exotic with respect to Laurentia, but the origin of the Chopawamsic terrane remains unclear. We use ICP-MS laser ablation geochronology of detrital zircons from samples in the eastern Blue Ridge, Western Piedmont, and Chopawamsic terranes, Virginia to better constrain provenance and tectonic affinity. Neoproterozoic to early Paleozoic Iapetan rift and passive margin deposits in the eastern Blue Ridge and western Piedmont are strongly unimodal age with 90% of zircons between 1,030 and 1,180 Ma; consistent with a local Laurentian source of Grenville-age. Detrital zircons from quartzite in the Shores mélange complex along the western boundary of the Chopawamsic terrane range from in age from 620 to 2,060 Ma with peak modes occurring at 1,150-1,200, 1,250-1,300 Ma, and 1,500-1,550 Ma. Ordovician volcanic and plutonic rocks in the Chopawamsic terrane are unconformably overlain by metasedimentary rocks of the Arvonia and Quantico formations. Based on fossil assemblages these units have been interpreted as late Ordovician successor basin deposits formed at the end of the Taconic orogeny. Basal quartzites of the Arvonia/Quantico formations are dominated by 440 to 470 Ma detrital zircons, with grains as young as 390 Ma and as old as 2,700 Ma; 1,000 to 1,200 Ma zircons are rare in these samples. The young zircons require a depositional age of early Devonian/late Silurian for the Arvonia/Quantico sequence. The paucity of 1,000 to 1,200 Ma zircons indicates the Chopawamsic terrane was not derived from a Laurentian source. Older Proterozoic zircons were likely sourced from a Gondwanan craton and the 1,500-1,550 Ma zircons may have been derived from the Cachoeirinha magmatic suite in the Rio Negro-Juruena province of the southwestern Amazon craton.