BEYOND ZIRCON: INCORPORATING DETRITAL K-FELDSPAR PB ISOTOPE ANALYSIS IN SEDIMENTARY PROVENANCE STUDIES – AN EXAMPLE FROM A LONG-LIVED EASTERN LAURENTIAN CLASTIC SYSTEM

Due to inherent limitations (differential fertility of source terranes; orogenic recycling) detrital zircon (DZ) geochronology may inform sedimentary processes more than it informs provenance. Use of additional detrital proxies is thus important. The common Pb isotope composition (²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb) of detrital K-feldspar (DKf) was measured on the same clastic units in southeastern Laurentia that were previously characterized by DZ and detrital monazite (DMz) for provenance analysis. The purpose is to test a model that invokes late Paleozoic recycling of sediment initially sourced from erosion of exhuming Grenvillian (1.3-1.0 Ga) basement in the Neoproterozoic. The approach takes advantage of the demonstrated difference in Pb isotope composition between Laurentian and Amazonian cratonic sources. Neoproterozoic samples sourced from southern Amazonia and central Laurentian basement that serve as controls on methodology plot within Pb isotope space characteristic of their respective sources. DKf in the Cryogenian Ocoee Supergroup in the southern Appalachian orogen falls within the field of Pb isotopic compositions defined by south-central Appalachian basement (SCAB). The latter in turn exhibits Pb isotope compositions characteristic of Amazonia because SCAB is exotic and was transferred to Laurentia from Amazonia during Rodinian assembly. In contrast, DKf in lower Cambrian arenite falls within the Laurentian field, indicating a change in the early Paleozoic sediment source from the Laurentian craton. DKf in lower Pennsylvanian lithic arenites of the central Appalachian basin exhibit Pb isotope compositions that fall within the SCAB field but at slightly more radiogenic isotopic compositions than inferred Ocoee sources. Incorporation of all provenance constraints requires an immediate source that is isotopically more radiogenic than the Ocoee, but similar in all other petrologic and geochronologic characteristics, e.g., other Cryogenian to Ediacaran units along strike in the Appalachian orogen (e.g., Lynchburg Group, central Blue Ridge). As was demonstrated using DMz in provenance analysis, the results further demonstrate the importance of employing multiple detrital mineral proxies for fully capturing the provenance history rather than using DZ geochronology alone.