Genesis of metapelitic migmatites in the Adirondack Mountains, New York

Abstract

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 d 18O 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 Ôwhole grainsÕ (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, d18 O (Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1-13.4&; n = 95, 10 lm spot). In contrast, garnet is typically unzoned and d 18O (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 d

Assessment of grain-scale homogeneity and equilibration of carbon and oxygen isotope compositions of minerals in carbonate-bearing metamorphic rocks by ion microprobe

“…Carbon and oxygen isotope zonation in metamorphic minerals and grain-to-grain variations in d 13 C and d 18 O, however, are known facts (Wada, 1988;Conrad, 1991, 1993;Graham, 1991, 1993;Kohn et al, 1993;Young and Rumble, 1993;Jamtveit and Hervig, 1994;Eiler et al, 1995a,b;Graham et al, 1998;Lewis et al, 1998;Satish-Kumar et al, 1998;Wada et al, 1998;Page et al, 2007Page et al, , 2010Bowman et al, 2009;Lancaster et al, 2009). A systematic survey of the extent of grain-scale carbon and oxygen isotope heterogeneity in metamorphic rocks, however, has waited for instrumentation that is capable of rapid, accurate, high-precision measurements of d 13 C and d 18 O at spatial scales smaller than normal grain size.…”

Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids

“…An energy offset of -40 eV, associated with an energy slit of 44 eV, was applied, so that most molecular interferences are significantly reduced. In previous studies, a mass-resolving power (MRP) of 5,000-8,000 was applied in order to resolve LREE oxide interferences on HREE atomic ions (Lancaster et al 2009;Page et al 2007). However, because the focal points for ions differ slightly according to mass, the 7 Li ?…”

“…Ca, Fe, U, and Th were analyzed as potential tracers of alteration. Zircons were analyzed for 23 trace elements (Li, P, Ca, Ti, V, Fe, Y, REE, Th, and U) using an IMS-1280 ion microprobe in the WiscSIMS Laboratory at the University of Wisconsin, Madison, with analytical procedures similar to those in Page et al (2007) and Lancaster et al (2009). A primary 16 O -beam with a current of 4 nA and a 10-kV accelerating voltage was defocused to a *25-lm-diameter spot.…”

High‐resolution geochemical record of fluid–rock interaction in a mid‐crustal shear zone: a comparative study of major element and oxygen isotope transport in garnet

“…, 2010), and changes in its trace element composition can be related to the growth or dissolution of important accessory minerals at specific metamorphic conditions (e.g. Rubatto & Hermann, 2007; Dutch & Hand, 2009; Lancaster et al. , 2009; Kelsey & Powell, 2011).…”