AGE AND ND ISOTOPIC COMPOSITION OF DETRITAL MONAZITE CRYSTALS: TECTONIC EVENTS RECORDED IN HIGH FIDELITY

A recent study compared the ages of detrital zircon versus detrital monazite from modern river alluvium in the southern Appalachians (Hietpas et al. 2010, Geology, v.38). The results from that work demonstrate that ages of detrital monazite more faithfully record the complex Appalachian Paleozoic tectonic record than detrital zircon ages which are strongly skewed towards Mesoproterozoic (Grenville) ages. The differences between the ages recorded by the two mineral proxies are interpreted to be the result of the significant differences in their physical and petrogenetic properties.

The potential of detrital monazite ages to capture past tectonic events was further assessed by analyzing six Pennsylvanian-Early Permian Appalachian foreland basin sandstones that were previously characterized by detrital zircon ages. The results of this study demonstrate that detrital monazite provides a higher fidelity Paleozoic signal compared to zircon. In fact, for two of the sandstones, detrital monazite recorded multiple orogenic events that were completely missed by detrital zircon.

In addition to utilizing detrital monazite ages, we measured the Nd isotopic composition of previously dated monazite crystals isolated from modern alluvium from the French Broad River, TN. Initial ε _Nd values of Paleozoic monazite grains range from -10 to -4 whereas Grenville-aged monazite has ε _Nd (1 Ga) of -6 to -2. But when epsilon values are determined at the same age the majority of the Grenvillian and Paleozoic monazite have overlapping Nd isotopic composition, suggesting an ultimate Grenville source for much of the Paleozoic monazite. However, some Paleozoic detrital monazite crystals have ε _Nd (T) distinct from Grenville grains indicating different sources were also involved in monazite genesis. Thus Nd isotopic composition of detrital monazite can be useful for identifying provenance that cannot be resolved with ages alone. These results have important implications for assessment of siliciclastic sedimentary source, constraining sediment depositional age, and reconstructing tectonic history.