Resetting of oxybarometers and oxygen isotope ratios in granulite facies orthogneisses during cooling and shearing, Adirondack Mountains, New York

Ian Cartwright, John W. Valley, Anne Marie Hazelwood

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The petrography, petrology, and oxygenisotope geochemistry of granulite-facies granitic and syenitic orthogneisses of the Diana and Stark complexes, Adirondack Mountains, New York, show that the extent and nature of resetting of isotopic and mineralogic systems is highly variable. There is a strong correlation between retrogression and shearing, and the rocks may be divided texturally into: (1) unsheared lithologies that preserve little-retrogressed pyroxene-or hornblendebearing peak-metamorphic mineralogies; and (2) sheared rocks that underwent retrogression, marked by the growth of late biotite, in centimetre-to metre-wide shear zones after the peak of metamorphism. Oxygen fugacities in the unsheared lithologies were estimated for reintegrated mineral compositions from magnetiteilmenite (Mt-Ilm) and ferrosilite-magnetic-quartz (Fs-Mt-Qtz) equilibria. Mt-Ilm yields log fO2Mt-Ilm values of-15.9 to-17.6 (0.6 to 1.3 log units below the fayalite-magnetite-quartz buffer, FMQ) and temperatures of 670-745°C that agree with those from other geothermometry and phase equilibria studies. These data suggest that, aside from oxyexsolution of ilmenite from magnetite, the Fe-Ti system underwent only minor resetting during cooling, and the Fe-Ti oxides yield good estimates of peak-metamorphic temperatures and fO2. In unsheared ilmenite + magnetite + orthopyroxene + quartz assemblages, values of log fO2Mt-Ilm are lower than log fO2Fs-Mt-Qtz by an average of 0.6 when the orthopyroxene activity model of Sack and Ghiorso is used. Minor resetting of the Fe-Ti oxides, analytical errors, and errors in the placement of end-member reactions probably account for this relatively small difference in fO2 values. Whole-rock δ18O values of unsheared Diana and Stark lithologies range from 4.0 to 10.3‰ reflecting pre-regional metamorphic oxygen-isotope ratios. Peak-metamorphic minerals preserve high-temperature oxygen-isotope fractionations, and, in many samples, the effective diffusion of oxygen in minerals ceased at higher temperatures than predicted from wet experimental diffusion data. These data suggest that the rocks did not contain an aqueous fluid phase during cooling. The combination of petrologic, isotopic, and textural data also permits a detailed study of shearing and retrogression. Ilmenites in the sheared lithologies underwent greater degrees of hematite loss than in the unsheared rocks, resulting in log fO2Mt-Ilm values as low as-24.1 (3.1 log units below FMQ) and Mt-Ilm temperatures that are up to 175°C below regional estimates. Sheared rocks also have higher δ18O values (up to 13.3‰). During shearing, δ18O values of biotite, K-feldspar, and magnetite reset readily, while the degree of isotopic resetting of quartz correlates with the intensity for recrystallization.

Original languageEnglish
Pages (from-to)208-225
Number of pages18
JournalContributions of Mineralogy and Petrology
Issue number2
Publication statusPublished - 1 Feb 1993

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