DETRITAL MONAZITE GEOCHRONOLOGY AND TEXTURES PROVE SEDIMENT RECYCLING IN APPALACHIAN CLASTIC SYSTEMS

Detrital zircon age spectra in Appalachian clastic wedges and modern river systems are dominated by late Mesoproterozoic (“Grenville”) age modes, with minor middle Paleozoic, Proterozoic, and Archean modes. The Grenville dominance, due primarily to zircon fertility of Grenville granitoids of eastern Laurentia, has also been interpreted as evidence of sedimentary recycling; a first cycle vs. a recycled derivation has also been debated for early to middle Proterozoic and late Archean age modes. Unfortunately, there are no definitive criteria for distinguishing first-cycle from recycled detrital zircon with ages determined by SIMS or LA-ICP-MS geochronology, which is a limitation of detrital zircon U-Pb geochronology for provenance studies. Our previous work on detrital monazite geochronology provides an alternative perspective to zircon on sedimentary provenance in modern and ancient clastic systems. It demonstrated that pelitic regional metamorphic rocks were contributing a significant component of detritus to late Paleozoic and modern Appalachian drainage systems. Micro-imaging of detrital monazite grains from Permo-Carboniferous (P-C) arenites in WV/VA/TN/AL (depositional age = 320-280 Ma), and comparison with inclusion-rich diagenetic monazite forming in late Neoproterozoic, low grade Walden Creek Gp. (WCG) metaclastic rocks, reveals that detrital diagenetic monazite sourced from late Neoproterozoic to mid. Ordovician sediments comprises ~1/3 of the detrital monazite age spectrum for the P-C arenites. Diagenetic monazites are riddled with inclusions, similar to WCG diagenetic monazite, but are rounded, consistent with a detrital origin. Diagenetic monazite grains have ages almost exclusively of 400-650 Ma, with a dominant age mode at 450 Ma, similar to the Walden Ck. samples, with only a few Grenville monazites. The diagenetic monazites must have been sourced from low grade, late Neoproterozoic to early Devonian sedimentary rocks, proving recycling. In contrast, rounded inclusion-free (most likely metamorphic) detrital monazite exhibits a lesser 450 Ma mode, a dominant 400 Ma mode, and significant ~350 ages, consistent with first cycle derivation from regional metapelites. A significant Grenville mode and scattered Paleoproterozoic/ Archean grains are equivocal as to recycling.