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1

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Stable isotopic and petrological constraints on scapolitization of the Whitestone meta-anorthosite, Grenville Province, Ontario (1992)

MOECHER, D. P. ; ESSENE, E. J. ; VALLEY, J. W.

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 10 (1992), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The Whitestone Anorthosite (WSA), located in the Central Gneiss Belt of the south-western Grenville Province, Ontario, exhibits a nearly concentric metamorphic envelope characterized by an increase in modal scapolite, hornblende, epidote and garnet, developed around a core of granulite facies clinopyroxene ± orthopyroxene ± garnet meta-anorthosite. Scapolite- and hornblende-bearing assemblages develop mainly at the expense of plagioclase and pyroxene within the envelope.Stable isotopic and petrological data for scapolite-bearing mineral assemblages within meta-anorthosite constrain the source of carbon responsible for CO3-scapolite formation and the extent of fluid/rock interaction between the anorthosite and adjacent lithologies. Stable isotopic data indicate increasing δ18O and δ13C from core to margin of the meta-anorthosite and for samples from the southern extension of the WSA, where it is ductilely deformed within the Parry Sound Shear Zone (PSSZ). The average δ18OSMOW value (whole rock) for the WSA core is 6.9‰, increasing to 11.5‰ where the WSA is in tectonic contact with marble breccia. The average δ13CPBD value of scapolite in meta-anorthosite from the centre of the WSA is -3.4‰, increasing to -0.5‰ at the eastern (marble) contact. Average values of δ13C for scapolite and whole-rock δ18O for samples from the shear zone are -1.0 and 8.0‰, respectively. Marbles have average δ18O and δ13C values of 19.2 and -0.4‰, respectively.The sulphate content of texturally primary scapolite decreases from the core of the WSA (XSO4= 0.48) to the eastern contact (≤0.05). Texturally late scapolite after plagioclase and garnet tends to be CO3-rich relative to texturally primary scapolite, and some scapolite grains show zoning in the anion site with CO3-enriched rims. Scapolite composition may vary at any scale from a single grain to outcrop.The pattern of isotopic enrichment in 13C and 18O preserved in the eastern margin of the WSA is consistent with marble as the major source of fluid contributing to the formation of the metamorphic envelope. The decrease in XSO4 and increase in XCO3 in scapolite toward the margin of the WSA indicate that the volatile content was reset by, or developed from, a CO2-bearing fluid. Assuming derivation of fluid from marble, minimum fluid/rock values at the margin of the WSA range from 0.03 for the least enriched, to 0.30 for the most isotopically enriched samples. Although marble is not found in immediate contact with samples of sheared meta-anorthosite from the PSSZ, a marble source is also consistent with the C and O isotope composition and anion chemistry of scapolite within these samples.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1992.tb00120.x

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High-temperature granulites (1990)

HAYOB, J. L. ; ESSENE, E. J. ; RUIZ, J.

[s.l.] : Nature Publishing Group

Nature 347 (1990), S. 133-134

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] HAYOB ETAL. REPLY - Harley and Harte and Barnicoat question our inference that granulite-facies paragneiss xenoliths from Quaternary eruptions in central Mexico preserve the highest regional metamor-phic temperatures yet recorded in deep-seated crustal rocks. Elsewhere, Harley2 has compiled P- T ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/347133b0

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Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists, Dora Maira Massif, western Alps (1993)

Sharp, Z. D. ; Essene, E. J. ; Hunziker, J. C.

Springer

Contributions to mineralogy and petrology 114 (1993), S. 1-12

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The δ18O(smow) values of the quartz (after coesite) (δ18O=8.1 to 8.6‰, n=6), phengite (6.2 to 6.4‰, n=3), kyanite (6.1‰, n=2), garnet (5.5 to 5.8‰, n=9), ellenbergerite (6.3‰, n=1) and rutile (3.3 to 3.6‰, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H2O, the a(H2O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X(CO2)〉0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are ∼680 °C/30 kb at a(H2O)=1.0 and are calculated to be ∼70°C higher at a(H2O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (δ18O=11.6‰) and garnet (δ18O=8.7‰) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (δD(SMOW)=-27 to-32‰), on secondary talc and chlorite rite after pyrope (δD=-39 to -44‰) and on the surrounding biotite (δD=-64‰) and phengite (δD=-44‰) gneiss. All phases appear to be in nearequilibrium. The very high δD values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar δD, but dissimilar δ18O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00307861

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U-Pb geochronology of the Grenville Orogen of Ontario and New York: constraints on ancient crustal tectonics (1993)

Mezger, K. ; Essene, E. J. ; Pluijm, B. A. ; [et al.]

Springer

Contributions to mineralogy and petrology 114 (1993), S. 13-26

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Based on lithological, structural and geophysical characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones. In order to constrain the timing of metamorphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites. In addition, U-Pb ages were obtained for monazites from metapelites and for a rutile from an amphibolite. These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen. Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals: Central Granulite Terrane: Adirondack Highlands: 1150 Ma; 1070–1050 Ma; 1030–1000 Ma Central Metasedimentary Belt: Adirondack Lowlands 1170–1130 Ma Frontenac domain 1175–1150 Ma Sharbot Lake domain ca. 1152 Ma Flzevir domain: 1240 Ma; 1060–1020 Ma Bancroft domain: ca. 1150 Ma; 1045–1030 Ma Central Gneiss Belt: ca. 1450 Ma; ca. 1150 Ma; 1100–1050 Ma Grenville Front Tectonic Zone ca. 1000 Ma. Combination of mineral ages with results from thermobarometry indicates that metamorphic pressures and temperatures recorded by thermobarometers were reached polychronously in the different domains that are separated by major shear zones. Some of these shear zones such as the Robertson Lake shear zone and the Carthage-Colton shear zone represent major tectonic boundaries. The Grenville Orogen is made up of a collage of crustal terranes that have distinct thermal and tectonic histories and that were accreted laterally by tectonic processes analogous to those observed along modern active continental margins. The subsequent history of the orogen is characterized by slow time-integrated cooling rates of 3±1°C/Ma and denudation rates of 120±40m/Ma.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00307862

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Ultra-high temperatures from oxygen isotope thermometry of a coesite-sanidine grospydite (1992)

Sharp, Z. D. ; Essene, E. J. ; Smyth, J. R.

Springer

Contributions to mineralogy and petrology 112 (1992), S. 358-370

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The oxygen isotope compositions of coesite, sanidine, kyanite, clinopyroxene and garnet were measured in an ultra-high pressure-temperature grospydite from the Roberts Victor kimberlite, South Africa. The δ18O values (per mil v. SMOW) of each phase and (1 σ) are as follows: coesite, 8.62 (0.31); sanidine, 8.31 (0.02); kyanite, 7.98 (0.08); pyroxene, 7.63 (0.11); garnet, 7.53 (0.03). In situ analyses of the coesite with the laser extraction system are δ18O=9.35 (0.08), n=4, demonstrating that the coesite is homogeneous. The coesite has partially inverted to polycrystalline quartz and the pyroxene is extensively altered during uplift. The larger scatter for the mineral separate coesite and pyroxene data may be due to partial reequilibration between the decompression-related breakdown products of these two phases. The anomalously high δ18O value of the grospydite (δ18Owholerock=7.7‰) is consistent with altered oceanic crust as a source rock. Temperature estimates from a linear regression of all the data to three different published calibrations correspond to an equilibrium temperature of 1310±80°C. The calculated isotopic pressure effect is to lower these estimates by about 40°C at 40 kb. The estimated temperature based on Al−Si disorder in sanidine is 1200±100°C and that from Fe−Mg exchange thermometry between garnet and clinopyroxene is 1100±50°C. Given the large errors associated with thermometry at such high temperatures, it is concluded that the xenolith equilibrated that 1200±100°C. Pressure estimates are 45±5 kb, based on dilution of the univariant equilibria albite = jadeite + coesite and 2 kyanite + 3 diopside = grossular + pyrope + 2coesite. Zoning in the outer 20 μm of the feldspar from Ab0.8 to Ab16 indicates rapid decompression to 25 kb or less. The isotopic temperature estimates are the highest ever obtained and combined with the high degree of Al−Si disorder in sanidine require rapid cooling from ultra-high temperatures. It is inferred that the xenolith was sampled at the time of equilibration, providing a point on the upper Cretaceous geotherm in the mantle below South Africa.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00310466

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