SHRIMP U-PB SYSTEMATICS OF ZIRCON FROM ANORTHOSITE, NELSONITE, AND ILMENITE-MAGNETITE ORE AT THE TAHAWUS MINE, ADIRONDACK MOUNTAINS, NY: EVIDENCE FOR A YOUNGER EPISODE OF EMPLACEMENT

The High Peaks region of the central Adirondacks is underlain primarily by anorthosite and related rocks (the AMCG suite) of the Marcy massif. Previous U-Pb geochronology on zircon extracted from several samples of Marcy anorthosite indicated an emplacement age of about 1.15 Ga, and subsequent metamorphism at about 1.05 Ga. During a USGS study of Adirondack iron deposits, we sampled ilmenite-magnetite ore, host anorthosite, and associated nelsonite (Fe-Ti oxide-apatite rock) from the historic Tahawus mine area for SHRIMP U-Pb geochronology. Ti-Fe ore and nelsonite are considered igneous components of anorthosite magmatism.

Two groups of zircon from anorthosite include: (1) heterogeneous, partially resorbed, oscillatory-zoned grains (ages of 1.21-1.05 Ga), and (2) whole grains exhibiting tortoise-shell (fine growth/sector) zoning (1037 ± 4 Ma). Both groups of grains are overgrown by 2 episodes of rims: (1) unzoned, light gray inner rims (1030 ± 5 Ma), and (2) unzoned, black outer rims (1010 ± 8 Ma). Low-U (20-90 ppm) zircon from Ti-Fe ore (with unusual striped CL zoning) has ages of 988 ± 5 Ma (cores) and 958 ± 5 Ma (rims). Very low-U (5-20 ppm) zircon from nelsonite (with similar striped zoning in CL) yields an age of 1020 ± 23 Ma (or possibly about 1030 Ma for cores and 1000 Ma for rims). Group 1 zircon contains numerous micro-mineral inclusions of plagioclase, quartz, K-feldspar, and amphibole, whereas Group 2 grains are mostly inclusion-free. SEM-EBSD measurements of Group 2 zircon indicate that all individual sectors within a single grain are identically oriented crystallographically. We conclude that Group 1 grains are xenocrystic (i.e. inherited), whereas Group 2 grains are igneous and date emplacement of the anorthosite. The rims formed during subsequent episodes of metamorphism. Lack of 1.05 Ga metamorphic zircon suggests that time of emplacement of the anorthosite was after the 1.05-1.09 Ga Ottawan orogeny.

The anorthosite at Tahawus is about 120 m.y. younger than other dated samples from the Marcy massif, including an anorthosite located about 15 km to the SSE. The extent of younger anorthosite in the Adirondacks (similar in age to the Valin, Château-Richer, St. Urbain, and Labrieville anorthosites of Québec, and Montpelier and Roseland anorthosites of VA) is unknown and warrants further investigation.