Stable isotopic characteristics and magmatic history of meta-igneous rocks, Adirondacks, NY

Abstract

The Adirondack Highlands contain dominantly meta-igneous rocks including the anorthosite-granite association and a wide variety of unrelated orthogneisses from gabbroic to granitic composition. The internal variation of oxygen isotopes within Adirondack meta-igneous suites related to the Marcy anorthosite shows that elevated δ^(18)O is a compositionally dependent characteristic of the protolith. Analysis of δ^(18)O in previously unsampled compositions in this suite has led to a stable isotopic, petrographic and major element description of a continuous range of rock types from two-pyroxene, oxide rich monzonitic gneiss to mesoperthite, quartz rich granite gneiss within the Tupper and Saranac sheets on the NW margin of the Marcy anorthosite. This expanded data set has allowed the formulation of a semi-quantitative model, based both in thermodynamics and mass balance, for the AFC magmatic evolution of this suite. A sampling traverse of the Snowy Mountain dome anorthosite and related halo of granitic gneiss in the southern Adirondack Highlands shows the concentric spatial distribution of δ^(18)O in granites around anorthosite, constrains the extent of interaction between anorthosite and granite, and provides an additional example of crustal interaction in the magmatic evolution of this suite. This evidence, combined with previous work (Morrison and Valley, 1988), documents extensive crustal interaction in region-wide anorthosite-granite plutonism prior to the peak of Grenville metamorphism.