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40560-B8
Basinwide Provenance of Appalachian Basin Acadian Orogenic Sediments from a Late Frasnian Time Slice
This project seeks to establish the basinwide provenance of the Appalachian basin Upper Devonian sediments within a narrow time slice. The project design called for sampling from New York to Tennessee within a range of sedimentary facies. Samples were collected from approximately 15 localities. Whole rock samples from all localities have been analyzed by ICP-MS and XRF for Pb isotopes, Si, Al, Ca, K, Na, Fe, Mg, Mn, Ti, Rare Earth Elements (REE), Sc, Zr, Cr, V, Y, Ni, Th, U, Pb and Th. Additionally, thin section petrographic analyses have been made on 23 sandstones from three proximal localities.
Results of petrographic analyses of three localities indicate sourcing of proximal sandstones from recycled orogenic terranes. These results are consistent with previous works. The shales and sands analyzed to this point, record an apparent range in the character of the source components to the basin. While all rock types at all of the locations sampled exhibit characteristics consistent with a distinctly Grenvillian component, there are a number of deviations which reflect the addition of both older (archean) and juvenile source components as well as evidence of a recycled orogenic source.
Trace element whole rock data from the northern, central and southern Appalachian Basin add insight into the nature of the source area. Some southern basin shales exhibit low Y/Ni and high Cr/V ratios possibly indicating the presence of chromite which would be an indicator for the presence of an ophiolitic component. Elevated Zr/Sc relative to Th/Sc in samples from throughout the basin is indicative of zircon enrichment through recycling.
Whole rock 206Pb/204Pb and 207Pb/204Pb values can be used to characterize the Pb isotopic characteristics of sources to the Devonian Appalachian Basin. Samples from the northern parts of the basin define a typical field for shales with a Grenville-type source with in the basin. Parts of the central and southern basin also reflect this Grenville-type source, however, a number of southern samples form an array that can best be described as Archean in nature. A large number of central basin shales form an array between the Grenville-type field and the Archean source suggesting significant mixing of these sources in the central part of the basin during the Devonian.
The 207Pb/204Pb ratio can also be used to detect the presence of an old (i.e., pre-Grenvillian) Pb component in these rocks. Due to the relatively short half-life of 235U as compared to 238U, a sample with a high 207Pb/204Pb (>15.7) is likely to contain old Pb which in this case is most likely associated with recycled, possibly Archean-aged, zircons. If this contention is correct, then the majority of the rocks sampled are derived from a Grenville-type source but, there is evidence of an old, pre-Grenvillian, recycled component in samples from the southern and central portion of the basin.
These results suggest the Acadian hinterland in the southern and central portion of the Devonian Appalachian Basin was composed of multiple components in addition to those with a Grenvillian character. This represents significant new information about the Acadian orogen. This is especially important because little is known of its nature, especially in the central and southern Appalachians where several diverse tectonic elements were available along the Laurentian margin to source the Acadian foreland basin.
