In situ granitoid genesis and dehydration in the Adirondacks
Penelope J. LancasterBin FuF. Zeb PageN. T. KitaM. E. BickfordBarbara M. HillJames M. McLellandJohn W. Valley
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The Adirondack Mountains, NY are ideal for studying melting and migmatites in conformable granitic leucosomes cutting melanocratic metasediments. Leucocratic bands range in thickness from mm- to dm-scale, and have average quartz:K-spar:plagioclase norms of 46:16:38 in the upper amphibolite facies NW Lowlands and 26:51:23 in the granulite facies SE Highlands. To determine whether these leucocratic bands were local melts of surrounding melanosomes or externally-derived intrusions, we have correlated cathodoluminescence (CL) imaging, in situ U-Pb geochronology by SHRIMP, in situ zircon δ18O measurements by CAMECA IMS 1280 ion microprobe, and metamorphic garnet δ18O measurements by laser fluorination at 9 locations. CL imaging indicates three populations of zircons in both regions: 1. relatively featureless rounded 'soccer balls' (metamorphic), and rhythmically zoned (igneous) cores truncated by either 2. discordantly zoned (igneous) or 3. unzoned (metamorphic) rims. The U-Pb ages confirm CL classification as either 'igneous' or 'metamorphic' and determine the timing of different events. Typical δ18O ion microprobe spot-to-spot reproducibility of zircon standards is ±0.14‰ (1SD). For zircons from 3 leucosomes from the NW, igneous cores average 7.7±2.2‰ (1SD, VSMOW, n=5), a single igneous rim is 8.2‰, and metamorphic rims and whole grains average 10.1±1.9‰ (n=14). In corresponding melanosomes, igneous zircon rims average 8.4±1.4‰ (n=3) and garnets average 10.5±0.5‰. Average zircon rim age is 1208±33Ma, while average metamorphic age is 1165±58Ma. For zircons from 6 leucosomes from the SE, igneous cores average 8.2±2.1‰ (n=7) and igneous rims average 11.6±0.6‰ (n=4); metamorphic rims and grains average 10.1±1.4‰ (n=36). In corresponding melanosomes, igneous zircon rims average 11.7±0.04‰ (n=2), while garnets average 11.5±1.4‰. Average zircon rim age is 1103±63Ma, while average metamorphic age is 1132±80Ma. Values of 'igneous' δ18O in leucocratic layers are unusually high for plutonic rocks, especially in the SE. These high δ18O values (>10‰) cannot represent nearby magmas and indicate melting of surrounding metapelites. Metamorphism and anatexis occurred concurrently, and dehydration by melting at 1.2-1.1Ga lead to low water activity during Ottawan granulite metamorphism at ~1050Ma.Keywords:
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Coordinated cathodoluminescence (CL) imaging and ion microprobe (SHRIMP and CAMECA 1280) analysis document micron-scale U-Pb-O isotope and trace element zoning in zircons from deep crust exposed to 80 m.y. of high temperature and pressure metamorphism. Three, along-strike paragneiss samples across the amphibolite to granulite facies transition in the Kapuskasing Uplift crustal cross-section in the Archean Superior province yield detrital, originally igneous zircon cores overgrown by progressively larger volumes of metamorphic zircon with increasing grade. The cores generally retain primary age (2.85±0.03 to 2.67±0.02 Ga), oxygen isotope (5.1 to 7.0‰) and trace element compositions similar to those reported for magmatic arc sources. Dark CL, metamorphic zircon rims record nearly continuous overgrowth events for ∼80 m.y. from 2.66±0.01 to 2.58±0.01 Ga during uppermost amphibolite to granulite facies regional metamorphism. These rims have significantly higher δ^18^O values (8.4 to 10.4‰) and trace element compositions quite distinct from those of the cores; these differences indicate that their δ^18^O and trace element compositions were not inherited from the igneous cores, consistent with extensive textural evidence for rim formation as metamorphic overgrowths. Multi-spot traverses record steep oxygen isotope discontinuities (4‰ over \<10 μm) at core-rim boundaries, confirming the extremely sluggish rates of volume diffusion of O in non-metamict zircon during extended (∼80 m.y.) granulite-grade metamorphism (peak T=750-800 °C) at substantial f(H~2~O) but water-undersaturated (fluid-absent) conditions. Likewise no evidence of significant diffusive exchange of δ^18^O could be detected along deformation microstructures such as annealed fractures in cores infilled with high δ^18^O zircon. Application of simple diffusion models to detailed δ^18^O profiles in a large number of zircon grains constrain maximum values of the diffusivity of oxygen in zircon (logD^Zrc^~ox~) to the range −27.5 to −26.4 m^2^/s. For the estimated 80 m.y. and 700 to 800 °C time-T window of rim formation, these maximum values are similar to or slower than values reported by Page and others (2007, 2010) and the experimentally-determined "dry" diffusivity of oxygen in zircon (Watson and Cherniak, 1997), but are markedly slower than the experimentally-determined "wet" diffusivity of oxygen in zircon (Watson and Cherniak, 1997). Fast diffusion of oxygen in zircon predicted by hydrothermal experiments may, in nature, require the presence of a hydrous fluid rather than a threshold value of f(H~2~O). Our test demonstrates that unrecrystallized metamorphosed igneous zircons and metamorphic zircons will retain the geochemical (U-Pb age, trace element and δ^18^O) record of their origin and evolution despite prolonged, high-grade metamorphism at significant f(H~2~O) but water under-saturated (fluid-absent) conditions. Such zircons, particularly those that exhibit δ^18^O zoning, are micron-scale records for the T-time-fluid interaction history of deep crustal rocks. Such records will not be preserved in less refractory phases and promise new insights into the processes of continent formation and evolution.
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The Adirondack Mountains, NY are ideal for studying melting and migmatites in conformable granitic leucosomes cutting melanocratic metasediments. Leucocratic bands range in thickness from mm- to dm-scale, and have average quartz:K-spar:plagioclase norms of 46:16:38 in the upper amphibolite facies NW Lowlands and 26:51:23 in the granulite facies SE Highlands. To determine whether these leucocratic bands were local melts of surrounding melanosomes or externally-derived intrusions, we have correlated cathodoluminescence (CL) imaging, in situ U-Pb geochronology by SHRIMP, in situ zircon δ18O measurements by CAMECA IMS 1280 ion microprobe, and metamorphic garnet δ18O measurements by laser fluorination at 9 locations. CL imaging indicates three populations of zircons in both regions: 1. relatively featureless rounded 'soccer balls' (metamorphic), and rhythmically zoned (igneous) cores truncated by either 2. discordantly zoned (igneous) or 3. unzoned (metamorphic) rims. The U-Pb ages confirm CL classification as either 'igneous' or 'metamorphic' and determine the timing of different events. Typical δ18O ion microprobe spot-to-spot reproducibility of zircon standards is ±0.14‰ (1SD). For zircons from 3 leucosomes from the NW, igneous cores average 7.7±2.2‰ (1SD, VSMOW, n=5), a single igneous rim is 8.2‰, and metamorphic rims and whole grains average 10.1±1.9‰ (n=14). In corresponding melanosomes, igneous zircon rims average 8.4±1.4‰ (n=3) and garnets average 10.5±0.5‰. Average zircon rim age is 1208±33Ma, while average metamorphic age is 1165±58Ma. For zircons from 6 leucosomes from the SE, igneous cores average 8.2±2.1‰ (n=7) and igneous rims average 11.6±0.6‰ (n=4); metamorphic rims and grains average 10.1±1.4‰ (n=36). In corresponding melanosomes, igneous zircon rims average 11.7±0.04‰ (n=2), while garnets average 11.5±1.4‰. Average zircon rim age is 1103±63Ma, while average metamorphic age is 1132±80Ma. Values of 'igneous' δ18O in leucocratic layers are unusually high for plutonic rocks, especially in the SE. These high δ18O values (>10‰) cannot represent nearby magmas and indicate melting of surrounding metapelites. Metamorphism and anatexis occurred concurrently, and dehydration by melting at 1.2-1.1Ga lead to low water activity during Ottawan granulite metamorphism at ~1050Ma.
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The rocks at Chimney Mountain provide a rare glimpse into primary intrusive relations and exceptionally well-preserved pre-Shawinigan metasedimentary rocks in the Adirondack Highlands despite a strong Ottawan thermal overprint. A near vertical contact between granite (ca. 1172 Ma) and a shallowly dipping and structurally intact sequence of quartzose to calc-silicate metasedimentary rocks is exposed on the southern flank of Chimney Mountain in the Central Adirondacks. The contact is marked by foliation truncation and a metasomatic aureole with randomly orientated porphyroblasts of enstatite rimmed by anthophyllite (max. 5 cm) and phlogopite (max. 2 cm), and a zone of granular, quartz-rich rock. The granite is non- to weakly foliated and has a shallow, north-plunging, mineral lineation as do the metasedimentary rocks. Zircons separated from a diopside-bearing quartzite (82% SiO2; 0.75 m thick) are of variable size (up to 400 μm), equant, and contain, on average, >1000 ppm uranium. Scanning electron microscope investigation indicates that there is little variation in a uniformly dark cathodoluminescence response, no discernible cores or rims, few inclusions, and partially faceted to round morphologies. Zircon U-Pb sensitive high-resolution ion microprobe (SHRIMP II) ages of 1042 ± 4 Ma and 1073 ± 15 Ma are coincident with Ottawan metamorphic ages from the Adirondack Highlands. Zircons from the intrusive granite yield large cores with typical anorthosite-mangerite-charnockite-granite (AMCG) ages (ca. 1171.6 ± 6.3 Ma) and sparse, thin younger rims (ca. 1060–1090 Ma) readily distinguishable by cathodoluminescence. Despite the younger zircon ages, the metasedimentary rocks and their fabric must predate the crosscutting granite. The thermal effect of the Ottawan event was likely enhanced by volatile fluxing and resulted in recrystallization and resetting of zircons in the metasedimentary rocks. However, it had limited effects on zircons in the granite and produced only thin metamorphic rims emphasizing the importance of local geochemical conditions to the response of zircon to metamorphism. Elzevirian or Shawinigan fabrics are preserved as the dominant foliation; the lineation and folding is likely late (post-1170 Ma) Shawinigan or Ottawan (ca. 1050 Ma). Titanites from the same metasedimentary sequence yield a range of 238U/206Pb ages from 969 to 1077 Ma, with a maximum probability age of 1035 Ma, similar to other titanites in the Adirondack Highlands. Ottawan paleotemperatures, estimated by zirconium in titanite thermometry, range from 787 to 818 °C.
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The Seve Nappe Complex exposed in the Kittelfjäll area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 °C and 780 °C, which is in agreement with the c. 774 °C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 °C and 726 °C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475–469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (LuN/GdN = 0.49–38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (LuN/GdN = 3.96–65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.
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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 δ 18 O 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, δ 18 O (Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μ m spot). In contrast, garnet is typically unzoned and δ 18 O (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 δ 18 O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ 18 O (Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ 18 O (Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ 18 O metasedimentary rocks.
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