High-Grade Metamorphism of Iron-Rich Bodies in Archean Gneiss, Wind River Mountains, Wyoming

Abstract

The Warm Springs field area is situated in the northern Wind River mountains and is composed predominantly of Archean gneiss. Within this terrain, two unusual iron-rich bodies crop out. These are believed to be high-grade regionally metamorphosed iron-formation. The quartzo-feldspathic gneiss shows evidence of two phases of folding. It is occasionally migmatitic. Small conformable pods of amphiboloite occur within the gneiss, indicating the regional metamorphic grade is now amphibolite facies. To the northeast of the area is the Warm Springs Pluton, composed of rocks of granitic to quartz-monzodioritic composition. Granitic dikes, plagioclase-rich pegmatites and quartz veins cut all these rock types. Paleozoic sediments unconformably overlie the gneiss and the pluton. The majority of the area is now covered by glacial drift. The northern iron-rich body, the Wildcat body, contains the highest grade assemblage consisting of olivine, orthopyroxene, clinopyroxene, quartz and magnetite. The equigranular, granoblastic texture indicates it is an equilibrium assemblage. The olivine and pyroxenes show retrograde reactions to amphiboles. This mineralogy indicates that the Wildcat body was metamorphosed to granulite facies. The southern body, the Spruce Creek body, contains more garnet and biotite than the Wildcat body indicating that it is more aluminous. Both pyroxenes occur but olivine is not seen. This body shows an unusual intergrowth of quartz and magnetite that is believed to be due to intense silicification of the iron-silicates. The assemblage olivine-orthopyroxene-clinopyroxene-quartzmagnetite is used for obtaining both fossil temperatures and pressures. Two pyroxene geothermometry gives a temperature range of 500-650°c and a zoning within the grains of 100°c. This is substantiated by other methods. The olivine, orthopyroxene and quartz geobarometer yields a pressure of 3.6-5.9 kbar for this temperature range. This temperature and pressure are within the amphibolite facies. Pigeonite, suggested by the intergrowth of orthopyroxene and clinopyroxene in the Wildcat body yield temperatures of 900°c and pressures of 8-9 kbar which indicate granulite facies metamorphism. The area has undergone two periods of metamorphism and several periods of deformation. The two iron-rich bodies crop out along a lineament and a magnetometer survey indicated other iron-rich bodies also conformable with this trend. It is suggested that the two bodies were originally part of one body, disrupted during the Late Precambrian.