Recycling is widely accepted to be pervasive in clastic sedimentary systems. Proving recycling occurred and quantifying the proportion of the recycled component is important for sedimentary provenance analysis and assessment of ultimate versus immediate sediment source in tectonic studies that test models for terrane collision and crustal assembly. The most widely applied single mineral provenance tool in use today—detrital zircon U-Pb geochronology—cannot distinguish first-cycle from recycled detrital zircon, and thus cannot test for sedimentary recycling without time intensive and low throughput alternative dating methods. In contrast to zircon, monazite’s ability to crystallize during sedimentary diagenetic and low-grade burial metamorphic events, and to survive subsequent erosion/transport events, can be used to prove recycling occurred in clastic rocks. Imaging and textural analysis of monazite grains from Neoproterozoic syn-rift and Pennsylvanian−Permian foreland basin arenites in the southern Appalachians permits distinction among monazite grain types: (1) inclusion-rich monazite formed during diagenesis or low-grade metamorphism in the host arenite; (2) detrital diagenetic (and/or low-grade) monazite formed in older clastic units in the sediment source region and subsequently re-transported; and (3) detrital monazite likely derived from crystalline rocks (mostly regional pelitic schists and gneisses) in the source region. Secondary ion mass spectrometry Th-Pb geochronology reveals that Neoproterozoic feldspathic arenites of the upper Ocoee Supergroup in the southern Appalachian orogen contain late Mesoproterozoic detrital metamorphic monazite grains derived from Grenville-age basement crystalline rocks, an inference supported by the presence in the same samples of Grenville detrital zircon U-Pb ages that match the dominant detrital monazite age modes at ca. 1050 and 1150 Ma. The upper Ocoee arenites also contain Cambrian to middle Ordovician aged, diagenetic/low-grade metamorphic monazite grains that precipitated and grew post-depositionally in the arenites, and are texturally distinct (inclusion-rich, anhedral) from the detrital metamorphic monazite grains in the same sample (inclusion free, equant sub-rounded single grains). The Pennsylvanian−Permian arenites contain two distinct textural types of monazite: (1) late Neoproterozoic to Devonian aged detrital diagenetic monazite grains, interpreted as such based on identical textures to the upper Ocoee diagenetic monazite; and (2) Precambrian to Paleozoic (as young as 315 Ma) detrital monazite most likely derived from Taconian and Mesoproterozoic (Grenville) metapelitic rocks in the source region. The detrital diagenetic monazite in the Pennsylvanian−Permian arenites, which comprise approximately a third of the dated grains, must have been derived from erosion of older (Neoproterozoic, at the oldest) sediments, thus proving sediment recycling. Detrital monazite, although less common than detrital zircon in clastic rocks, provides critical additional insights into sedimentary provenance and recycling based on simple petrographic observations, which cannot be made using the standard approach of U-Pb zircon geochronology in provenance studies.
Proof of recycling in clastic sedimentary systems from textural analysis and geochronology of detrital monazite: Implications for detrital mineral provenance analysis
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D.P. Moecher, E.A. Kelly, J. Hietpas, S.D. Samson; Proof of recycling in clastic sedimentary systems from textural analysis and geochronology of detrital monazite: Implications for detrital mineral provenance analysis. GSA Bulletin doi: https://doi.org/10.1130/B31947.1
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