SEARCHING FOR MEANING IN A MOUNTAIN OF DETRITAL ZIRCON AND MONAZITE AGES AND TEXTURES FROM SOUTHEASTERN LAURENTIA

Thousands of detrital zircon (DZ) and monazite (DM) U-Th-Pb ages from Mesoproterozoic through Paleozoic clastic sequences provide insight into (1) Grenville orogenesis; (2) Midcontinent rifting; (3) post-Rodinian exhumation and breakup; (4) evolving sedimentary dispersal systems; and (5) nearly 1 Ga of sediment recycling. The “Grenville doublet” (or “quadruplet” resolvable in large datasets) of Ottawan-Shawinigan (Rigolet/Elzevirian/Pre-Elzevirian) zircon magmatic and metamorphic ages fingerprints the fully exhumed Grenville orogen by the mid-late Neoproterozoic. In DZ studies across Laurentia the presence or absence of the doublet defines pre-, syn-, or post-Ottawan deposition. Analytical precision is critical for resolving the two modes, e.g., some studies citing an eastern Laurentian Grenville provenance show one major age ‘hump’ spanning ca. 1.3-0.9 Ga. The doublet dominates Neoproterozoic continental rift sequences (e.g., Ocoee Supergroup) and margin-slope-rise sequences, and their metamorphosed equivalents. It is absent from some paragneisses in the Great Smoky Mtns. basement complex that contain major pre-Elzevirian and 1.6-2.0 Ga modes documenting an early Meso- and late Paleoproterozoic provenance component. CL zoning permits distinction between Ottawan metamorphic vs. magmatic DZ. Correction of discordant analyses for 450 Ma (Taconian) metamorphic Pb loss expands the number of useful ages for confirming Proterozoic DZ grains. The Neoproterozoic sequences and basal Cambrian arenites in eastern Laurentia contain the bulk of the Grenville DZ that was recycled through successive Appalachian orogenic phases. The Midcontinent rift magmatic interval conveniently splits the doublet implying temporary cessation of Grenville orogenesis. The long debated late Grenville foreland and East Continent rift basins are distinguished based on DZ age modes and interpreted in the context of pre- and post-Ottawan exhumation. DM sourced from regional Taconic and Acadian metapelites and S-type granitoids can be distinguished texturally from “detrital diagenetic monazite”, the bulk of the latter forming during Cambrian-Middle Ordovician burial and low-grade metamorphism in Ocoee-equivalent units, proving Neoproterozoic through Phanerozoic recycling of Grenville zircon.