In situ granitoid genesis and dehydration in the Adirondacks

Oxygen isotope ratios and REE concentrations provide independent tests of competing models of injection vs. anatexis for the origin of migmatites from amphibolite- and granulite-facies metasedimentary rocks of the Adirondack Mountains, NY. Values of δ¹⁸O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2Ga) are surrounded by overgrowths crystallised during the Grenville orogenic cycle (~1.2–1.0Ga). Cathodoluminescence imaging records three populations of zircons: 1. featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either 2. discordant rhythmically zoned (igneous) or 3. unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite-facies NW Adirondacks and the granulite-facies SE Adirondacks, δ¹⁸O_(Zrc) values in overgrowths and whole zircons are highly variable for metamorphic zircon (6.1–13.4‰; n=95, 10μm spot, 2SE=±0.4‰). In contrast, garnets are typically unzoned and δ¹⁸O_(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrologic estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircons. The unusually high δ¹⁸O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ¹⁸O_(Zrc)18O(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ¹⁸O metasedimentary rocks.