Crystallization Conditions of the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming |
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Authors: | FUHRMAN MIRIAM L; FROST B RONALD; LINDSLEY DONALD H |
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Institution: | 1Department of Earth and Space Sciences, State University of New York Stony Brook, NY 11794-2100
2Department of Geology and Geophysics, University of Wyoming Laramie WY 82071 |
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Abstract: | The Sybille Monzosyenite, associated with the Laramie AnorthositeComplex, consists of rocks ranging in composition from monzogabbroto monzosyenite. There is a continuous range of mineral compositionswith plagioclase varying from An45 to An25 and olivine fromFa75 to Fa98 Strongly ternary (Or, Ab, An all > 10 mol%)feldsparspresently mesoperthitesare found in allrock types and define a continuous trend also in the feldsparternary system. The mineral compositions suggest that the rocktypes of the Sybille Monzosyenite could be part of a singledifferentiation sequence; contamination of the later units byassimilation of, or admixture of partial melts from, countryrock is also likely. Original magmatic temperatures of approximately9501050?C are indicated by estimated original compositionsfor pyroxenes and feldspars; pressure was near 3 kb, as indicatedby the most magnesian olivine that coexists with quartz. Oxygenfugacity of crystallization is estimated as 1.5 to 2.0 log unitsbelow FMQ by using the displaced equilibrium: SiO2 + 2Fe2TiO3= 2FeTiO3 + Fe2SiO4. Such oxygen fugacities are consistent withthe occurrence of graphite and CO2-rich fluid inclusions, whichsuggest that crystallization took place in the presence of aCO2 vapor phase. Temperatures indicated by the present mineralassemblages show that all geothermometers used were reset duringcooling, first by intergrain and then by intragrain processes. |
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