Exploiting detrital monazite geochronology and Sm-Nd isotopic tracer signatures for provenance investigations.

 

Jack Hietpas1, Scott Samson1 and David Moecher2

1Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244

2 Dept. Earth & Env. Sci., University of Kentucky, Lexington, KY 40506

 

A recent study compared the ages of detrital zircon versus detrital monazite from modern river alluvium in the southern Appalachians (Hietpas et al. 2010). The results from this investigation have shown that ages of detrital monazite more faithfully record the complex Appalachian Paleozoic record as opposed to detrital zircon which are strongly skewed towards Mesoproterozoic (Grenville) ages. The differences between the ages recorded by the two proxies are interpreted to be the result of the significant differences in the physical and petrogenetic properties of the minerals. 

            The provenance potential of detrital monazite ages 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 major orogenic events that were completely missed by detrital zircon. 

            In addition to utilizing detrital monazite ages, we measured in situ Sm-Nd isotopic signatures of previously dated monazite isolated from modern river alluvium (Fischer et al. 2011).  Initial 143/144Nd ratios of Paleozoic grains range from -10 to -4 epsilon units, Grenville-aged grains span -6 to -2 epsilon units.  The majority of the Paleozoic grains plot within the “evolution window” of the Grenville grains.  However, some grains fall outside this window, indicating these grains were derived not simply from metamorphosed Grenville material.  Initial 143/144Nd ratios of Paleozoic grains provided additional information that identified separate sediment donors regions that could not be resolved with ages alone.  The results of this study have important implications for accurate assessment of sources of siliciclastic sediments, constraining sediment depositional age and the tectonic conclusions inferred by these assessments.