PROOF OF RECYCLING IN CLASTIC SEDIMENTARY SYSTEMS FROM TEXTURAL OBSERVATIONS AND TH-PB SIMS GEOCHRONOLOGY OF DIAGENETIC DETRITAL MONAZITE: CHANNELING ONE’S INNER BOB DOTT AND IMPLICATIONS FOR DETRITAL MINERAL PROVENANCE STUDIES

One focus of R.H. Dott’s research was generation of mature quartz arenites and the potential role of recycling, which is widely accepted to be pervasive in clastic systems. Proving recycling occurred and quantifying the proportion of the recycled component is important for sedimentary provenance analysis. The most widely applied single mineral provenance tool - detrital zircon (DZ) U-Pb geochronology - cannot distinguish first-cycle from recycled detrital zircon. In contrast, 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. SEM imaging and textural analysis of monazite grains from Neoproterozoic syn-rift and Penn-Permian southern Appalachian foreland basin arenites permits distinction among monazite textural grain types: (1) inclusion-rich monazite formed during diagenesis or low-grade metamorphism in the host arenite; (2) detrital diagenetic/low-grade monazite formed in older clastic units in the sediment source region and subsequently eroded and re-transported; and (3) detrital monazite likely derived from crystalline rocks (mostly regional metapelites) in the source region. Th-Pb SIMS geochronology reveals that Neoproterozoic arenites contain late Mesoproterozoic detrital metamorphic monazite derived from Grenville-age basement crystalline rocks, an inference supported by the presence in the same samples of Grenville DZ U-Pb ages that match the detrital monazite age modes at ca. 1050 and 1150 Ma. The Neoproterozoic arenites also contain Cambrian to middle Ordovician, diagenetic/low-grade metamorphic monazite grains that are texturally distinct (inclusion-rich, anhedral) from the detrital metamorphic monazite grains (inclusion free, equant sub-rounded single grains). The Penn-Permian arenites contain Neoproterozoic to Devonian detrital diagenetic monazite grains, and Paleozoic (as young as 315 Ma) and Precambrian detrital metamorphic monazite. The detrital diagenetic monazite in the Permo-Carboniferous 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.