Isotopic Constraints on Emplacement Age of Anorthositic Rocks of the Marcy Masiff, Adirondack Mts., New York

The igneous emplacement age of anorthositic rocks comprising the Marcy massif have been constrained by fixing minimum and maximum ages for their intrusion. Minimum ages have been fixed by: (a) U-Pb dating of cores of air abraded zircons from the anorthosite (>1113 Ma), (b) U-Pb dating of baddeleyite within the anorthosite (>1087 Ma), (c) U-Pb dating of baddeleyite from an olivine metagabbro crosscutting Marcy massif anorthosite (>1109 Ma), (d) U-Pb zircon dating of jotunitic rocks crosscutting the Marcy anorthosite massif ($1143 \pm 31 Ma$). Maximum emplacement ages are based upon: (a) sheets of anorthositic rock crosscutting mangeritic-charnockitic gneiss dated by U-Pb zircon at$1125 \pm 10 Ma$,$1134 \pm 4 Ma$, and$1135 \pm 43 Ma$, and (b) anorthosite-related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite. These results constrain emplacement of the Marcy anorthosite to 1113-1138 Ma with the most probable age in the interval 1125-1135 Ma. This time of intrusion is coincident with that of the granitoid envelope enclosing the anorthosite massif and indicates that, together, these rocks represent a bimodal magmatic complex.