34 In the Field or in the Lab: Revealing Hidden Signals from Minerals

Tuesday, 16 April 2024
8:00 AM - 12:00 PM
Swannanoa

DETRITAL MONAZITE GEOCHRONOLOGY AND REE GEOCHEMISTRY IN PIEDMONT PLACER HEAVY-MINERAL ASSEMBLAGES, NORTHWESTERN SOUTH CAROLINA: TESTING METHODS FOR DETERMINATION OF PROVENANCE OF REE AND TI CRITICAL MINERAL DEPOSITS

Paper number: 34-3
Program: Theme Sessions
Time: 8:25 AM – 8:45 AM

Authors

Abstract

The USGS demonstrated the REE critical mineral potential of the southeast US Coastal Plain. High concentrations of REEs occur in sediments and strata southeast of the Fall Line and in some drainages flowing out of the Piedmont and Blue Ridge (BR). Questions remain regarding the age and lithologies of source terranes. We applied high-throughput (40 sec/date) and high-spatial resolution (10 micron spot) U-Pb dating and REE analysis via laser ablation split stream ICP-MS of detrital monazite (Mnz) to develop protocols for determining provenance in regional drainage systems using a proxy other than detrital zircon (Zrc). First-order streams of the Pacolet and Broad River drainages in northwestern South Carolina (Chesnee and Cowpens quadrangles) contain heavy-mineral suites dominated by Ilm (30-80%) and Mnz (10-50%), with lesser Ru, Zrc, and Sil/Ky. These assemblages correlate with immediately underlying lithologies of the eastern Inner Piedmont (Cat Square terrane), mostly muscovite schist and biotite gneiss. U-Pb dating and REE analysis of detrital Mnz provide constraints on provenance of the entire heavy-mineral assemblage and metamorphic age of bedrock. Dates for Mnz in three panned heavy-mineral concentrates define highly-resolved age modes of 320 (Alleghanian), 360 and 375 (Neo-Acadian), and 425 Ma (late Salinian?). Single Mnz grains (100-400 mm dia.) are chemically zoned in Th but chronologically homogeneous. REE concentrations normalized to upper continental crust show: (1) constant light REE values; (2) marked negative Eu anomalies; and (3) a wide range in HREE values (Gd/Lu = 10-1100). In-situ dating and analysis of Mnz in amphibolite-facies metapelites from bedrock units of the BR and Inner Piedmont yield ages that are dominantly Taconian (440-460 Ma) and Neo-Acadian (360 Ma), respectively. Mnz closer to the Brevard Fault Zone is Alleghanian (330 Ma). Eu anomalies are lower and Gd/Lu ratios are not as high (10-200) in basement samples as in eastern Piedmont detrital Mnz. These results, and previous detrital Mnz Th-Pb dating on French Broad River alluvium northwest of the BR, prove that detrital Mnz dating is more suitable for immediate provenance analysis of Mnz and co-produced Ilm than detrital Zrc, which is recycled from Grenvillian basement that sourced BR and Piedmont sedimentary protoliths.

Geological Society of America Abstracts with Programs. Vol. 56, No. 2

doi: 10.1130/abs/2024SE-398140

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