Eric J. Essene

Closure temperatures of the Sm---Nd system in metamorphic garnets

Abstract Garnet-whole rock and garnet-mineral isochrons were determined on granulite facies gneisses and amphibolites from the Archean Pikwitonei Granulite Domain of the Superior Province, and the Proterozoic Central Gneiss Belt and Adirondack Highlands of the Grenville orogen. The Sm—Nd ages obtained from Archean garnets 0.1–0.5 cm in length are 30–110 Ma younger than the U—Pb ages obtained on the same garnets and also younger than the time of the last regional metamorphism, as determined by the growth ages of the youngest metamorphic garnets and zircons. Similarly, the Sm—Nd ages obtained from Proterozoic garnets with a diameter of 0.1–5 cm are younger than the time of the last regional metamorphism and similar or younger than cooling ages obtained on sphenes from the same sample or from the same geologic setting. Only the core of a garnet with a diameter of ca. 30 cm and without abundant inclusions may record the time of garnet growth. Comparison of the Sm—Nd ages with other geochronologic data and temperature estimates leads to the conclusion that the closure temperature for the Sm—Nd system in garnets analyzed in this study is ca. 600 ± 30°C. Only garnets with radii much larger than 5 cm may record Sm—Nd growth ages in upper amphibolite facies rocks from slowly cooled terranes. Garnets from higher grade terranes yield cooling ages that define the retrograde history of metamorphic terranes.

Calculation and application of clinopyroxene-garnet-plagioclase-quartz geobarometers

Recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive Gibbs free energies for hedenbergite and pyrope. These were used to calculate the geobarometric equilibriaHedenbergite+Anorthite=Grossular+Almandine +Quartz: “HD barometer”,Diopside+Anorthite=Grossular+Pyrope+Quartz: “DI barometer”.We have compared the pressures obtained from these equilibria for garnet-clinopyroxene-orthopyroxene-plagioclasequartz assemblages with the geobarometerFerrosilite+Anorthite=Almandine+Grossular+Quartz: “FS barometer”.Pressures calculated for 68 samples containing the above assemblage from a variety of high grade metamorphic terranes indicate that, in general, the HD and DI barometers yield values that are in good agreement with the FS barometer, and that the three barometers are generally consistent with constraints from aluminosilicate occurrences. However, in some samples the HD barometer yields pressures up to 2 kbar greater than constraints imposed by the presence of an aluminosilicate phase. Relative to the FS barometer, the HD barometer overestimates pressure by an average of 0.2±1.0 (1σ) kbar and the DI barometer underestimates pressure by an average of 0.6±1.6 (1σ) kbar. The pressure discrepancies for the HD and DI barometers are likely to be a result of imprecision in thermodynamic data and activity models for silicates, and not a result of resetting of the clinopyroxene equilibria. The relative imprecision of the DI barometer relative to the FS barometer results from overestimates of pressure by the DI and FS barometers in Fe-rich and Mg-rich systems, respectively. Application of the HD and DI barometers to high grade Cpx-Gt-Pg-Qz assemblages yields pressures that are generally consistent with other petrologic constraints and geobarometers. It is concluded that the HD and DI barometers can place reasonable constraints on pressure (±1 kbar relative to the FS barometer) if not extrapolated to mineral assemblages whose compositions are extremely far removed from the end member system for which the barometers were calibrated.

Clay mineral thermometry; a critical perspective

Diagenetic clay minerals usually occur as heterogeneous assemblages of submicroscopic layers consisting of different structure types such as illite, smectite and chlorite, with variable composition within a given structure type, and with highly variable concentrations of imperfections. The dimensions of mixed-layering, the semi-coherent to coherent nature of the structures across the layering, and compositional heterogeneity occur at a scale well below that of an individual thermodynamic phase. These relations imply that most clays are not distinct minerals or phases, and that assemblages of clays in shales and mudstones are incompatible with the phase rule. Such relations are better evaluated in terms of the formation of metastable materials with each small unit having unique chemical properties, rather than as a small number of stable homogeneous phases. Consequently, treatment of most clay minerals in terms of equilibrium stability with either a thermodynamic or experimental approach is subject to error.Chemical reactions involving most clay minerals are best understood with kinetic models. These involve a great variety of parameters such as time, fluid/rock ratio, deformation history, nature of starting materials and transformation mechanisms, as well as the variables, such as temperature, pressure and composition, that are commonly used to define equilibrium. Solubility experiments on the stabilities of clay minerals are unlikely to attain equilibrium at low temperatures. Moreover, the activity of soluble species may be controlled by surface equilibria, or by absorbed or exchangeable cations. Interpretations of available experiments on the solubility of illite vs. other mineral assemblages are in violation of Schreinemakers’ rules and indicate lack of equilibrium.Predictable sequences of clay minerals as a function of temperature are best understood through the Ostwald step rule, in which clay mineral assemblages undergo reactions in response to kinetic factors that represent reaction progress rather than an approach to equilibrium. Currently used clay mineral thermometers (illite crystallinity, smectite/illite reaction, chlorite composition) are not based on equilibrium reactions. Such systems are not accurate thermometers and therefore have questionable utility.

Significance of the Nova Brasilândia metasedimentary belt in western Brazil: Redefining the Mesoproterozoic boundary of the Amazon craton

(� 910 Ma). The NBMB marks the Mesoproterozoic limit of the SW Amazon craton. The discordance of the NBMB to the NNW structural trend of the younger Aguapeo´ belt (200 km SE of NBMB), together with marked differences between the two belts in sedimentary environment, metamorphic grade, and timing of deformation, signify that these two belts are not geologically continuous. The ‘‘Grenvillian’’ deformation recorded by the NBMB belt marks the final docking of the Amazon craton and Paragua craton within the Rodinia framework. The Aguapeo´ belt, in contrast, seems to record only limited deformation internal to the Paragua craton. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 3660 Mineralogy and Petrology: Metamorphic petrology; 1035 Geochemistry: Geochronology; 8025 Structural Geology: Mesoscopic fabrics; 9360 Information Related to Geographic Region: South America; KEYWORDS: Rodinia, Grenville mobile belt, Amazon craton, Paragua craton, geochronology, P-T path. Citation: Tohver, E., B. van der Pluijm, K. Mezger, E. Essene, J. Scandolara, and G. Rizzotto (2004), Significance of the Nova ...

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

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 MaCentral Metasedimentary Belt:Adirondack Lowlands 1170–1130 MaFrontenac domain 1175–1150 MaSharbot Lake domain ca. 1152 MaFlzevir domain: 1240 Ma; 1060–1020 MaBancroft domain: ca. 1150 Ma; 1045–1030 MaCentral Gneiss Belt: ca. 1450 Ma; ca. 1150 Ma; 1100–1050 MaGrenville FrontTectonic 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.

Feldspar and Oxide Thermometry of Granulites in the Adirondack Highlands

AbstractThermometry of regionally metamorphosed granulites of the Adirondack Highlands has been undertaken using feldspar and iron-titanium-oxide equilibria. Electron microprobe analyses of 20 coexisting oligoclase (An18–30) and microcline perthite (Or57–87) pairs from charnockites and granitic gneisses give KD[Na/(Na+Ca+K]plag/[Na/(Na+Ca+K)]or = 2–3 yielding temperatures of 650 ° to 750 ° C in comparison to Secks (1971) experimental and Stormers (1975) calculated temperatures for inferred pressures of 8 kilobars. Microprobe analyses of 10 coexisting titaniferous magnetite (ulvöspinelss 16–45) and ilmenite (hematitess 4.7–6.5) pairs from the Marcy massif anorthosite and related gabbros give temperatures of 620 ° to 800 ° C in comparison to Buddington and Lindsleys (1964) experimental data. Oxygen fugacities buffered by this assemblage range between 10−20 and 10−16 and always lie within 10+1 of the f

Cooling and inferred uplift/erosion history of the Grenville Orogen, Ontario: Constraints from 40Ar/39Ar thermochronology

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THE ROLE OF INCLUSIONS IN U-PB AND SM-ND GARNET GEOCHRONOLOGY : STEPWISE DISSOLUTION EXPERIMENTS AND TRACE URANIUM MAPPING BY FISSION TRACK ANALYSIS

buffered by fayalite-magnetite-quartz. Exsolved albite in alkali feldspar and ilmenite (oxidized ulvöspinel lamellae) must be reintegrated to infer metamorphic temperatures. Both thermometers give internally consistent, reproducible and geologically reasonable results. The inferred 750 ° and 700 ° C isotherms wrap around the anorthosite massif in roughly concentric circles. Maximum metamorphic temperatures (790 ± 50 ° C) occur between Saranac Lake and Tupper Lake, New York.