SHRIMP U-PB AGES OF ZIRCON AND MONAZITE, AND GARNET LU-HF AGES IN ANORTHOSITE OF THE MARCY MASSIF, ADIRONDACK HIGHLANDS, NY: RECOGNIZING INHERITED, IGNEOUS, AND METAMORPHIC COMPONENTS

Previous geochronology studies of the Marcy massif, Adirondack Highlands, NY, concluded that all of the anorthosite crystallized at about 1155 Ma, and was contemporaneous with rocks of the mangerite/charnockite/granite (MCG) magmatic suite. This interpretation was based on SHRIMP U-Pb spot analyses of zircon and mutual cross-cutting relations. In this model, the oldest coherent zircon age groups are interpreted to represent crystallization ages, whereas younger ages are due to growth of metamorphic zircon or Pb loss.

We have taken a new look at several samples of Marcy Anorthosite, using high resolution CL imaging, large data sets (ca. 50 SHRIMP analyses per sample), trace element analyses of dated zircon, and geochronology of other accessory phases (monazite, garnet, and titanite). In the westernmost Marcy massif (about 4 km east of Tupper Lake), igneous zircon from a 0.4 m thick granitoid dike that intruded anorthosite is 1165 ± 5 Ma, providing a minimum emplacement age of anorthosite there; metamorphic monazite from this sample is 1020 ± 3 Ma. Anhedral to subhedral oscillatory-zoned zircon from two samples of anorthosite from the central and southern parts of the Marcy massif yield SHRIMP U-Pb ages of about 1200-1040 Ma. The youngest coherent ages groups are 1039 ± 7 and 1039 ± 11 Ma. Lu-Hf ages of metamorphic garnet from these samples are 1021.9 ± 2.7 and 1022.3 ± 2.9 Ma, similar to the monazite age, but 15-20 m.y. younger than the youngest zircon ages. If the young zircon is igneous in origin, these crystallization ages are similar to 1.04 Ga anorthosite in Québec (Valin, Château-Richer, St. Urbain, and Labrieville) and Virginia (Montpelier and Roseland), and imply that (1) there is a significant, previously unrecognized, component of inherited zircon (~1200-1050 Ma), and (2) there were multiple episodes of anorthosite emplacement spanning at least 125 m.y. An alternative interpretation is that the crystallization age of the anorthosite is about 1155 Ma, and that the young zircon and garnet (1.04 and 1.02 Ga, respectively) formed by unrelated metamorphic reactions. Criteria currently under investigation for distinguishing these hypotheses include trace element geochemical data, CL zoning patterns, morphologic characteristics of zircon, plus geochronology of associated trace minerals.