NEW SHRIMP U-PB EVIDENCE FOR MULTIPLE AGES OF ANORTHOSITE EMPLACEMENT, MARCY MASSIF, ADIRONDACKS, NY

New age data preclude the previous interpretation that all anorthosite of the Marcy massif, Adirondacks, NY, was emplaced at about 1.155 Ga. Instead, new zircon SHRIMP ages indicate that (1) much of the western lobe of the massif is composed of massive, undeformed 1.04-1.02 Ga anorthosite (Marcy facies), and (2) the eastern lobe, plus a thin to broad, discontinuous western and southeastern border zone, is composed primarily of deformed anorthosite (Whiteface facies) that crystallized at about 1.16 to 1.15 Ga.

Utilizing multiple SHRIMP analyses per single coarse zircon grain (n=15-22), U-Pb data for some Whiteface facies samples form somewhat scattered arrays along Concordia between about 1.16 and 1.00 Ga. This method allows identification of relatively undisturbed areas within grains and calculation of accurate crystallization ages. This range of ages, only from cores, suggests mobility of radiogenic Pb, probably caused by heat related to emplacement of nearby 1.04 Ga anorthosite.

Zircon from four samples of massive to weakly (flow?) foliated anorthosite from the western lobe (from near Saranac Lake, Middle Saranac Lake, Tahawus, and west of North Hudson) are anhedral, have unusual oscillatory zoning, and are interpreted as igneous. Selected ²⁰⁷Pb/²⁰⁶Pb data from these grains yielded weighted average ages of 1038 ± 10 (rims at 1024 ± 8), 1038 ± 7, 1037 ± 5 Ma, and 1040 ± 11 Ma, respectively, with little or no evidence of inheritance. A new sample of massive anorthosite from near Paul Smiths yielded a surprisingly young age of 1023 ± 5 Ma, identical to Lu-Hf ages from five samples of garnet from anorthosite throughout the massif. In the eastern lobe, zircon from a new sample of weakly foliated anorthosite from Slide Brook (east of Keene) yielded an older age of 1153 ± 7 Ma, similar to new ages from Whiteface, Woolen Mill, and Stockwell Mtn. The youngest member of this age group (1125 ± 8 Ma) is recorded by zircon from anorthosite at Exit 29 on the Northway. The 1.16-1.02 Ga range of ages documents a previously unknown complex history of at least two pulses of anorthosite emplacement in the Adirondacks.