TESTING FOR SEDIMENTARY RECYCLING USING DETRITAL MONAZITE AND ZIRCON U-TH/PB AND U-TH/HE “DOUBLE-DOUBLE DATING” IN PENNSYLVANIAN SANDSTONES OF THE CENTRAL APPALACHIAN BASIN

Recognition of sediment recycling is critical for provenance studies, which assume a direct path from sediment source to depositional basin. Zircon double dating identifies the proportion of recycled sediment, which cannot be constrained using solely U-Pb detrital zircon (DZ) ages as the latter yield the ultimate age of the DZ, not its immediate source age. Also, zircon cannot record prior sediment transport, burial, or diagenetic histories because of its refractory nature. Here we present an approach for using crystallization ages (U-Pb and Th-Pb) and exhumation/cooling ages (U-Th/He) for both DZ and detrital monazite (DM) to test for recycling (“double-double dating”). The extent of sediment recycling can be quantified using the lag time (∆t = t_U-Pb – t_U-Th/He), which increases in the case of recycling (> ca. 300 m.y.). Furthermore, the presence of “detrital diagenetic monazite”, which records a prior sediment diagenetic event, proves recycling (Moecher et. al., 2018). This technique is applied to Alleghenian clastic wedge arenites of eastern Kentucky. Samples include the lower Pennsylvanian Grundy Fm. lithic arenite and the Corbin Sandstone quartz arenite (laterally equivalent to the Grundy Fm.). Results for the compositionally mature Corbin (more distal) and compositionally immature Grundy (proximal) include: (1) DZs from both are dominated by the Grenville Ottawan-Shawinigan “age doublet” (ca. 1000–1060 Ma and 1140–1180 Ma respectively) present in virtually all eastern Laurentia Phanerozoic DZ age spectra. The Corbin contains more older ages (1.35-1.45, 1.6-1.8, 2.7-3.7 Ga) than the Grundy consistent with more cratonic provenance and recycling; (2) In contrast to DZ, DM ages in both units are dominated by Paleozoic ages (Taconian, Acadian, and Neo-Acadian) indicating the Grenville DZs are overestimating the proportion of Grenville provenance (“Grenville zircon fertility”) and are likely to be recycled from Paleozoic strata. Preliminary U-Th/He thermochronology of Grenville DZ from the Grundy and Corbin show cooling through the He closure temperature (~180°C) is tied to Taconian, Acadian, and Neo-Acadian exhumation (∆t > 600 Ma). This cooling history requires that the DZs were deposited in Taconic, Acadian, or Neo-Acadian foreland basin systems and subsequently recycled during the Alleghenian.