MONAZITE AGES IN AND FERTILITY OF CRYSTALLINE BASEMENT ROCKS OF THE EASTERN INNER PIEDMONT PROVINCE

The southeastern United States has been known for over a century as a potential resource for rare earth elements due to the abundance of monazite in sediments and crystalline basement rocks. However, sedimentary evolution of the monazite from bedrock to placer deposit and the provenance of the monazite remain untested. This study examined samples collected from the basement rocks of the Piedmont terrane in South Carolina and western North Carolina including peraluminous granites, metagranites, mica schists, and biotite gneisses, and heavy-mineral concentrates of first-order streams draining from these rocks, for age comparison and monazite abundance. Monazite grains were analyzed for U/Pb isotope ratios using LASS-ICP-MS, resulting in approximately 2,700 dates. These analyses illustrate the dominance of Neo-Acadian ages in all samples with a secondary Alleghenian age mode. There is a minor Acadian age mode found primarily in metagranite samples and one mica schist sample. A few monazite grains exhibited two to three age zones, but most are chronologically homogeneous. The lack of Taconian-aged monazite, which is dominant in the Blue Ridge terranes, provides a distinct contrast in monazite age signatures of potential monazite source terranes, which will enhance clarity for interpretations of the provenance of the Coastal Plain and trunk river sediments in future studies. Heavy mineral concentrates indicate an abundance of monazite (3.9 - 44.5% of heavy mineral grains) plus variable amounts of ilmenite, zircon, rutile, garnet, and aluminosilicates. The concentration of these minerals is important in determining the economic value of the monazite placer deposits in the event that they are developed as rare earth element resources.