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 IN EASTERN KENTUCKY

Recognition of sediment recycling is critical for sedimentary provenance studies, which assume a direct path from sediment source to depositional basin. Double-double dating identifies the proportion of recycled sediment, which cannot be obtained using U-Pb detrital zircon (DZ) ages alone. Here we present an approach for using crystallization ages (U-Pb and Th-Pb) and exhumation/cooling ages (U-Th/He) for DZ and detrital monazite (DM). The extent of sediment recycling can be quantified using the lag time (∆t), which increases in the case of recycling (~> 300 Ma). Furthermore, “detrital diagenetic monazite” grains should be present in samples for which ∆t indicates recycling. The technique is applied to Pennsylvanian sandstones of the Alleghenian clastic wedge within the Central Appalachian Basin of eastern Kentucky. The Alleghenian clastic wedge is a logical system for testing recycling, as four major collisional events (Grenville, Taconic, Acadian and Alleghenian orogenies) likely reworked the continental margin and recycled clastic sediment. Samples analyzed include the Grundy Formation, a medium-to-fine grained lithic arenite, and the Corbin Sandstone, a medium-to-fine grained quartzarenite, which is laterally equivalent and contained within the Grundy Fm. Preliminary results for the mature Corbin Ss. (more distal) and the time equivalent, immature Grundy Fm. (proximal) include: (1) Detrital zircons from the Grundy Fm. have two well-defined age modes from Ottowan and Shawinigan Grenville magmatic events (ca. 1000-1060 Ma and 1140-1180 Ma respectively) as well as lower age modes interpreted to be sourced from Eastern Granite-Rhyolite Province (ca. 1350-1450 Ma); (2) detrital zircon U-Pb ages from the Corbin Ss. are temporally more expansive and indicate sources from Grenville granites as well as DZ sourced from Taconic (450 Ma), Yavapai-Mazatzal (ca. 1500-1700 Ma), and the Kenoran (ca. 2700 Ma) orogenies. The Corbin is interpreted to have DZ sourced from the mid-continent as well as from Appalachian sources, with the oldest grain dated at 3765 Ma. As there were likely few exposed Proterozoic and Archean sources during the time of Corbin deposition, DZ of these ages are likely to be recycled. Th-Pb geochronology on DM as well as U-Th/He thermochronology will help further test this hyponthesis.