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Journal of Petrology | Volume 29 | Number 3 | Pages 699-729 | 1988
© Oxford University Press 1988

research-article

Crystallization Conditions of the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming

MIRIAM L. FUHRMAN1, B. RONALD FROST2 and DONALD H. LINDSLEY1

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

Received September 30, 1987; Revised February 2, 1988; ABSTRACT

The Sybille Monzosyenite, associated with the Laramie Anorthosite Complex, consists of rocks ranging in composition from monzogabbro to monzosyenite. There is a continuous range of mineral compositions with plagioclase varying from An45 to An25 and olivine from Fa75 to Fa98 Strongly ternary (Or, Ab, An all > 10 mol%) feldspars–presently mesoperthites–are found in all rock types and define a continuous trend also in the feldspar ternary system. The mineral compositions suggest that the rock types of the Sybille Monzosyenite could be part of a single differentiation sequence; contamination of the later units by assimilation of, or admixture of partial melts from, country rock is also likely. Original magmatic temperatures of approximately 950–1050°C are indicated by estimated original compositions for pyroxenes and feldspars; pressure was near 3 kb, as indicated by the most magnesian olivine that coexists with quartz. Oxygen fugacity of crystallization is estimated as 1.5 to 2.0 log units below FMQ by using the displaced equilibrium: SiO2 + 2Fe2TiO3 = 2FeTiO3 + Fe2SiO4. Such oxygen fugacities are consistent with the occurrence of graphite and CO2-rich fluid inclusions, which suggest that crystallization took place in the presence of a CO2 vapor phase. Temperatures indicated by the present mineral assemblages show that all geothermometers used were reset during cooling, first by intergrain and then by intragrain processes.


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