Genesis of metapelitic migmatites in the Adirondack Mts., New York
Oxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. 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 (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (∼1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) 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 zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is 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, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ18O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ18O(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ18O(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ18O metasedimentary rocks.