SHRIMP II GEOCHRONOLOGY OF THE ADIRONDACK AMCG SUITE, I: EMPLACEMENT CHRONOLOGY OF ANORTHOSITES AND GABBROS

Previous attempts to directly date the emplacement of Adirondack anorthosites and gabbros encountered problems with either a paucity of zircon, or inheritance and/or metamorphic overprinting. Here, we present results of a targeted study employing detailed backscattered electron (BSE) and cathodoluminescence (CL) imaging followed by in situ ion probe (SHRIMP) microanalysis on primary zircons from pegmatitic anorthosite or gabbro. This approach utilizes high analytical spatial resolution (5-30µm) and uniquely resolves the confusion and disagreement that has resulted from historical isotope dilution methods of multigrain, single grain or even attempted dissections of single growth-domains of grains.

AMCG zircons range up to a few mm in length and, internally, can display spectacular fine to broad, concentric, oscillatory and/or sector zoning revealed under BSE and CL. Moreover, they locally contain minor inclusions of qz±kf±pl±ap±bio; all of these being typical of primary, igneous zircon from evolved residual magmas in equilibrium with plagioclase-rich cumulate. Metamorphic zircon occurs either as distinct, structureless, metamorphic rims irregularly embaying igneous cores, or as discrete, multifaceted, equant grains (Bickford et al., 2002).

New SHRIMP U-Pb results for primary, magmatic zircon cores from across the Marcy massif (MM) indicate anorthosite emplacement as early as 1161 ± 12 Ma (2s) at Indian Carry in the Saranac Lakes area, and 1160 ± 15 Ma at Jay. Anorthosite in the southern MM yields a primary igneous age of 1153 ± 11 Ma (Blue Ridge Highway). In the eastern MM near Elizabethtown, Woolen Mill gabbro and mutually crosscutting Marcy anorthosite yield igneous ages between 1154 ± 9 and 1151 ± 6 Ma, respectively (Clechenko et al., 2002). North of Schroon Lake, a crosscutting coronitic metagabbro retains igneous zircons which crystallized at 1150 ± 14 Ma; emplacement of the host anorthosite of the southernmost MM must therefore predate this. Minor intrusions extend the known ages of basic plutonism and diking to as young as ca. 1140 Ma. Zircon ages younger than this are demonstrably related to new growth, recrystallization or variable Pb-loss during locally intense Ottawan metamorphism (to ca. 1000 Ma; Bickford et al., 2002). All of these results establish a ca. 1150 Ma age for the Adirondack AMCG suite.