DETRITAL MONAZITE AGES: A TECTONIC RECONSTRUCTION TOOL FOR PAST OROGENIC EVENTS?

Single mineral analyses as provenance proxies have become popular in the past decade because of their potential to provide information related to sedimentary basin evolution, tectonic reconstructions, and paleogeographic comparisons. Although different minerals have been examined in sedimentary provenance studies the vast majority have relied solely on U-Pb ages of detrital zircon. However, because zircon is extremely resistant to physical and chemical weathering it can survive multiple sedimentary cycles and thus can cause potential confusion between proximal and ultimate sources. In contrast, monazite is much less likely to be multiply recycled and thus may introduce less bias in provenance analysis. Recent studies of alluvium from the French Broad River, North Carolina, have demonstrated the potential that detrital monazite has over detrital zircon in recording the tectonic signal of the three major Appalachian orogenies (Taconian, Acadian, and Alleghanian). But further investigation is needed to fully examine the utility of detrital monazite geochronology for tectonic reconstruction studies. Thus, we have collected alluvium from near the confluence of the Ohio and Mississippi rivers with the goal of dating detrital monazite crystals. Our goal is to (1) obtain U-Th-Pb ages for detrital monazite crystals to compare to published detrital zircon U-Pb dates and (2) further test the utility of dating detrital monazite as an accurate tool for the reconstruction of tectonic events. The Ohio River, a major tributary to the Mississippi River, drains, in part, crustal regions of the central and Southern Appalachians. Thus it would be expected that a reasonable proportion of detrital minerals should yield Ordovician, Devonian and Permian ages. Current zircon data, however, show an overwhelming proportion of Mesoproterozoic (1.2-1.0 Ga) grains. We will test if this is also the case for monazite or if their ages more accurately reflect the timing of the main Appalachian orogenies. Monazite ages will be determined using in situ U-Th-Pb dating techniques followed by analysis of Nd isotopic characteristics. Results of this study will have important implications for utilizing monazite as a tectonic reconstruction tool, for identifying past metamorphic events, and for determining sediment provenance.