Evidence for multiple recycling in Neoproterozoic through Pennsylvanian sedimentary rocks of the Central Appalachian Basin

Abstract

The petrography of sandstones and neodymium (Nd) isotope compositions of mudstones from the central Appalachians indicate changing provenances during the late Neoproterozoic through Pennsylvanian evolution of eastern North America. In contrast, sandstones from all stratigraphic units are dominated by Grenvillian (950-1250 Ma) zircons. Neoproterozoic to Cambrian rift-related sandstones are mostly feldspathic arenites and contain detrital zircons that are exclusively Grenvillian. The associated mudstones have remarkably uniform initial Nd isotope compositions (εNd = -4.4 to -5.4 at the time of sedimentation) that also are consistent with a Grenville provenance. Cambrian passive margin sandstones are quartz arenites that contain, in addition to Grenvillian zircons, significant numbers of early Mesoproterozoic and late Paleoproterozoic (1300-1800 Ma) zircons. Intercalated mudstones have variable and low initial εNd values of -6.3 to -15.1 that, in common with the detrital zircons, indicate derivation from a mixture of Grenvillian and older crustal provinces to the west. Sandstones from the overlying foreland basin clastic wedges typically are lithic arenites. Those of Taconian age contain both sedimentary and metamorphic (polycrystalline quartz and chlorite schist) rock fragments, whereas Acadian and Alleghanian sandstones are dominated by metamorphic rock fragments (quartz-muscovite and quartz-chlorite schist, detrital mica, and polycrystalline quartz). Although detrital zircons are mainly Grenvillian, a few younger (Taconian) zircons are present in the clastic wedge sandstones, and, significantly, a Lower Pennsylvanian sandstone contains up to 10% Archean zircons of probable Superior Province heritage. Mudstones from the Taconian, Acadian, and Alleghanian clastic wedges have uniform Nd isotopic compositions (initial εNd = -7.7 to -9.0) that are consistent with recycling of the lower part of the passive margin prism with an admixture of new, freshly eroded Grenville basement. Sandstone petrography and detrital zircon geochronology indicate that Taconian-age sandstones and conglomerates were recycled from older passive margin sedimentary rocks that were uplifted in an accretionary prism in front of the approaching magmatic arc and that Acadian and Alleghanian sandstones were derived from deeper levels of this foldthrust complex. The rarity of Neoproterozoic and Paleozoic zircons in the clastic wedge sandstones is attributed to the presence of the accretionary prism as a tectonic highland between the magmatic arcs and the foreland basins.