U-PB DETRITAL ZIRCON PROVENANCE AND MINERALOGY OF CRETACEOUS TO MODERN HEAVY MINERAL SANDS WITHIN ATLANTIC COASTAL PLAIN SEDIMENTS OF THE SOUTHEASTERN US

Atlantic Coastal Plain (ACP) heavy mineral sand (HMS) deposits are a primary source of critical commodities Ti (ilmenite and rutile) and Zr (zircon) and potential source of REEs (monazite and xenotime). We use new U-Pb zircon and automated mineralogy data to better understand regional-scale sediment pathways and how they impact compositions of HMS deposits within the ACP. The study focuses on Cretaceous to Pliocene sediments proximal to the Fall Zone from MD to AL and modern beach sands from VA to FL.

New detrital zircon (DZ) analyses of ~55 samples are dominated by Grenville (~1.3-0.9 Ga) and Appalachian dates (~480-290 Ma). The samples have similar DZ age modes, but in varying proportions that broadly fall into 3 categories: 1) Grenville dominant or Grenville > Appalachian, 2) mixed Grenville and Appalachian, and 3) Appalachian >> Grenville mixed Grenville and Appalachian. Along the Fall Zone, DZ ages are, from NE to SW, Grenville dominant in MD, mixed in southern NC, Appalachian dominant in SC and GA, and again Grenville dominant in AL. There is a gradual transition in DZ age populations from majority Grenville to Appalachian between NC and SC and back to Grenville near the GA/AL border. Along the coast, DZs are Grenville dominant in VA and NC and mixed/Appalachian dominant from SC to FL. Along the coast the transition from Grenville dominant DZ to mixed is between NC and SC and appears to be abrupt. Additional sample analyses will fill in data gaps that further refine these transitions.

The DZ results show distinct provenance shifts in sediments along the Fall Zone, with perhaps more sourced from Paleozoic arc terranes in NC, SC, and GA, and a limited degree of orogen-parallel longshore transport. Along the coasts, broad mixing is apparent but the sharp shift in DZ signatures between NC and SC may reflect changes in longshore transport systems in that region, perhaps associated with the Cape Fear Arch. Such changes are consistent with variations observed via airborne radiometric data.

Preliminary mineralogy of HMS sample separates is highly variable, but typically dominated by ilmenite (8%->90%), rutile (3%-45%), and zircon (<1% to 44%). Other variably abundant HMS species include monazite, xenotime, staurolite, kyanite, and garnet. Further analyses of these data will provide complementary limits on sedimentary provenance.