LEAD ISOTOPIC EVIDENCE FOR A NON-GRENVILLIAN COMPONENT IN THE SOUTH-CENTRAL ACADIAN HINTERLAND

Currently, the nature of the Devonian Appalachian Acadian hinterland is poorly known. By the time of that event, terranes had been accreted to the Laurentian continental margin, of Grenville age, during Ordovician orogenisis related to the closing of Iapetus. Possible sources of Acadian sediment, then, include the Mesoproterozoic Laurentian margin and recycled sediments deposited on that margin during the opening of Iapetus. These sediments potentially carry a signature from a large part of the craton, and were affected by Early Paleozoic orogenisis. Additional sources include terranes accreted during Early Paleozoic orogenisis, sediment deposited in the Early Paleozoic foreland basin created by terrane accretion (possibly carrying their own diverse signature), and the landmass or landmasses that collided with Laurentia and caused the Acadian orogeny itself. Here we present Pb isotope results from a sequence of Upper Devonian distal shales deposited during the latest Frasnian and earliest Famennian stages near Big Stone Gap in southwestern Virginia that suggest the presence of at least two distinct Pb isotopic components. These rocks are part of a larger investigation to characterize the Acadian hinterland.

Whole rock Pb isotope data were collected from a set of shale samples with a range in total organic carbon (TOC) content. By analyzing samples with this range it will be possible to detect any potential disturbances to the provenance signature as a result of black shale processes. Whole rock 206Pb/204Pb and 207Pb/204Pb ratios range from 19.21 to 23.44 and 15.58 to 15.91 respectively. The large range in 206Pb/204Pb compositions is likely due to the addition of uranium to samples with a high TOC content at the time of deposition that has led to resetting of U-Pb isotope system. The range in 207Pb/204Pb ratios is due to at least two distinct isotopic source components one of which is likely Grenville in age.

These results suggest the Acadian hinterland in the south-central portion of the Devonian Appalachian Basin was composed of at least two components. The prolonged tectonic history of the eastern margin of North America suggests that the Acadian hinterland likely contained a range of possible sources in addition to the Grenville-type rocks and recycled sediments that make up the bulk of the Appalachian uplift.