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

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Authors:Cartwright, Ian; Valley, John W.; Hazelwood, Anne Marie
Author Affiliations:Primary:
Monash University, Victorian Institute of Earth and Planetary Sciences, Clayton, Victoria, Australia
Other:
University of Wisconsin, United States
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 113(2), p.208-225. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1993
Note:In English. 85 refs.; illus. incl. 2 plates, 8 tables, geol. sketch map
Summary:The petrography, petrology and oxygen-isotope geochemistry of granulite-facies granitic and syenitic orthogneisses of the Diana and Stark complexes, Adirondack Mts, show that the extent and nature of resetting of isotopic and mineralogical systems are 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 hornblende-bearing peak-metamorphic mineralogies, and 2) sheared rocks that underwent retrogression, marked by the growth of late biotite, in cm- to m-wide shear zones after the peak of metamorphism. function signO2 in the unsheared lithologies were estimated for reintegrated compositions from magnetite-ilmenite (Mt-Ilm) and ferrosilite-magnetite-quartz (Fs-Mt-Q) equilibria. Mt-Ilm yields log function signO2Mt- Ilm values of -15.9 to -17.6 (0.6-1.3 log units below the FMQ buffer) and T of 670-745 degrees C that agree with those from other geothermometry and phase equilibria studies. These data suggest that, apart from oxy-exsolution 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 T and function signO2. In unsheared ilmenite + magnetite + orthopyroxene + quartz assemblages, values of log function signO2Mt-Ilm are lower than log function signO2Fs-Mt-Q by an average of 0.6 when a published orthopyroxene activity model is used. Minor resetting of the Fe-Ti oxides, analytical errors, and errors in the placement of end-member reactions probably account for this small difference in function signO2 values. Whole-rock delta18O values of unsheared Diana and Stark lithologies range 4.0-10.3 per mille, reflecting pre-regional metamorphic O-isotope ratios. Peak-metamorphic minerals preserve high-T O-isotope fractionations, and, in many samples, the effective diffusion of O in minerals ceased at higher T 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 petrological, isotopic and textural data also permits a detailed study of shearing and retrogression. [P.Br.]
Subjects:Age; Chain silicates; Cooling; Facies; Framework silicates; Fugacity; Geochemistry; Geologic barometry; Gneisses; Granulite facies; IGCP; Iron; Isotopes; Magnetite; Metals; Metamorphic rocks; Metamorphism; Mineral assemblages; Models; O-18/O-16; Orthogneiss; Orthopyroxene; Oxides; Oxygen; Petrography; Petrology; Phase equilibria; Pyroxene group; Quartz; Retrograde metamorphism; Shear zones; Silica minerals; Silicates; Stable isotopes; Titanium; Adirondack Mountains; Hamilton County New York; Herkimer County New York; Jefferson County New York; Lewis County New York; New York; Saint Lawrence County New York; United States; Diana Complex; Fractionation; Northeastern New York; Stark Complex
Coordinates:N435000 N444000 W0754000 W0755400
Abstract Numbers:93M/3162
Record ID:1993027051
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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