Paper No. 6
Presentation Time: 8:00 AM-12:00 PM
SHRIMP II GEOCHRONOLOGY OF THE ADIRONDACK AMCG SUITE, II: GRANITOIDS, GABBROS, AND CROSSCUTTING DIKES
New SHRIMP zircon ages are reported for: a) charnockites and mangerites adjacent to the Marcy anorthosite massif (MM), b) a coronitic metagabbro, c)an anorthositic pegmatite, and d) a clinopyroxene-rich dike, all intrusive into the MM. The charnockitic rocks border the massif near Tupper Lake, Bloomingdale, and Minerva. Prior multigrain ages for the first two were 1134 ± 14 Ma and 1135 ± 43 Ma, respectively. All three granitoids contain xenocrysts of blue-gray andesine plucked from anorthosite, for which their ages set a minimum. New SHRIMP analyses of the charnockites yield average weighted ages of 1169 ± 9 Ma at Tupper Lake, 1157 ± 18 Ma at Bloomingdale, and 1159 ± 12 Ma at Minerva. The average of these three ages is 1164 ± 13 Ma, in agreement with the ca 1155 Ma SHRIMP ages recently reported for the MM (Clechenko et al., 2002, Hamilton et al., 2002) and establishes the essentially coeval character of the suite. In addition, these ages are indistinguishable from other SHRIMP age determinations on AMCG charnockites from Gore Mt (1155 ± 6 Ma), Snowy Mt (1177 ± 22 Ma), and Diana Complex (1154 ± 17 Ma). The average age of all six of these charnockites is 1159 ± 9 Ma, which is equivalent to new SHRIMP ages of ca 1155 Ma for the Marcy anorthosite (Hamilton et al., 2002). SHRIMP analyses on a coronitic metagabbro crosscutting the MM north of Schroon Lake yields a weighted average age of 1141 ± 11 Ma. Similar gabbroic rocks are widely associated with Adirondack anorthosite massifs and are clearly part of the AMCG suite. The Schroon Lake age is in close agreement with an age of 1151 ± 11 Ma recently reported for the Woolen Mill gabbro that crosscuts the MM near Elizabethtown (Clechenko et al, 2002). In the Ausable River at Jay, a crosscutting pegmatitic gabbroic anorthosite in a shear zone yields a SHRIMP age of 1151 ± 13 Ma and a comb-textured clinopyroxene-plagioclase dike yields a poorly constrained age of 1139 ± 89 Ma. All of these ages are similar to the SHRIMP II age of 1155 ± 9 Ma obtained for an AMCG ferrodiorite dike crosscutting the Oregon Dome anorthosite massif. Taken together, these 11 ages establish a well constrained ca 1155 Ma age for the Adirondack AMCG suite and the Marcy massif. Proposals that the Marcy anorthosite was emplaced at ca 1040 Ma are inconsistent with these, and other, data and are without foundation.