DETRITAL MONAZITE AGES FROM MISSISSIPPI RIVER ALLUVIUM – DISPLAYS FURTHER UTILITY AS A SUCCESSFUL PROVENANCE INDICATOR

In the past decade many provenance studies have relied extensively on the use of detrital zircon ages as indicators on regional geology. However, because of the physical and chemical resiliency of zircon detrital grains can be recycled multiple times the ages often record that of the original source but not of the proximal sediment source. This potential bias can be a serious confounding aspect of the use of zircon as an accurate recorder of sediment provenance. This has been well demonstrated in Modern alluvium from rivers draining in the southern Appalachians where the majority of detrital zircon yields 1.2-1.0 Ga ages. Major Paleozoic orogenic events, such as the Acadian and Alleghanian orogenies, are not recorded by detrital zircon in these river systems.

In contrast to zircon, monazite forms over a much broader temperature range and is less physically resilient. These qualities point to monazite as a potentially better indicator of past tectonic events as well as a more reliable indicator of proximal sources of sediment. Previous studies have examined the age of detrital monazite from alluvium within rivers draining Appalachian terranes of very well characterized ages. These studies have shown that monazite recorded a much more complete record of Appalachian tectonic events while still recording the presence of Grenville-age crust. We have further pursued monazite’s provenance potential by examining detrital monazites from the alluvium of the Mississippi River. As one of North America’s largest drainages, the Mississippi provides an ideal location for the further monazite provenance research. Samples were collected from the mouth of the Mississippi as well as its major tributaries. We report here on the U-Th-Pb ages of monazites sampled from 3 localities along the river. Each alluvium sample contains Grenville age grains, but unlike the zircon record monazite also reveals the signals of the Taconic (470-440 Ma), Acadian (420-380 Ma), and to a lesser degree, the Alleghanian (330-290 Ma) orogeny. The results of this work demonstrate further that monazite ages appear to be an excellent tool for provenance and for recording past tectonic events. Detrital monazite ages may prove useful in the investigation of a wide variety of geologic problems.