DETRITAL ZIRCON PROVENANCE AND MINERALOGY OF CRETACEOUS TO MODERN HEAVY MINERAL SANDS OF THE ATLANTIC COASTAL PLAIN, SOUTHEASTERN USA

Atlantic Coastal Plain (ACP) heavy mineral sand (HMS) deposits are a primary source of critical mineral commodities Ti (ilmenite and rutile) and Zr (zircon) and potential source of REEs (monazite and xenotime). A growing database of detrital mineral geochronology and mineralogy of Cretaceous to modern HMS in the southeast US ACP, from Maryland to Alabama, elucidate regional-scale sediment sources, their transport pathways, and how they impact compositions of HMS deposits.

Detrital zircon (DZ) analyses are dominated by Grenville (ca. 1300-900 Ma) and Appalachian (ca. 480-290 Ma) age modes but vary in proportion along the ACP. Along the Fall Zone (ACP unconformity) from NE to SW, DZ ages in Maryland are Grenville dominant, mixed Grenville-Appalachian in southern North Carolina, Appalachian dominant in South Carolina and Georgia, and back to Grenville dominant in Alabama. These transitions are abrupt at the catchment scale. Along the coast, DZ are Grenville dominant in Virginia and North Carolina, and mixed-to-Appalachian dominant from South Carolina to Florida. The age mode transitions along the coast are distinctive but appear more gradual, possibly due to long-shore mixing.

Mineralogy of HMS heavy mineral fractions is highly variable, but typically dominated by ilmenite (8%->90%), rutile (3%-45%), and zircon (<1% to 44%). Other variably abundant but important HMS species include monazite, xenotime, staurolite, kyanite, and garnet. Geographical variability in Fall Zone mineralogy appears dependent on localized sources. For instance, monazite/xenotime is much more abundant in southern North Carolina-South Carolina and central Georgia than in northern North Carolina and Virginia. For modern HMS, drainage patterns and geomorphic features appear to control certain mineralogy (e.g. garnet is most abundant along the South Carolina coastline) whereas other mineral phases seem to be well-mixed broadly (e.g. staurolite and kyanite).

Overall, HMS depositional processes seem to vary between the Fall Zone and modern shoreline, likely due to margin type at time of deposition—the Fall Zone was probably a fluvial dominated, high topographic gradient margin.

Correlations between geochronology and mineralogy are not always present but are especially apparent between Appalachian dominant DZ age modes and and monazite/xenotime abundance in HMS.