Beneath the Stillwater Complex: Petrology and geochemistry of quartz-plagioclasecordierite (or garnet)-orthopyroxene-biotite ± spinel hornfels, Mountain View area, Montana
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Jennifer A. Thomson
Abstract
A 50 m drill core (core no. 383-334) collected from the contact metamorphic aureole in the Mouat Ni-Cu prospect in the Mountain View area of Stillwater County, Montana, exhibits evidence of in situ fluid-absent biotite dehydration melting and local back reaction of melts derived from metasediments and metavolcanics. The drill core was investigated to characterize the mineral assemblages and their textures, to search for evidence of partial melting, and to determine mineral and whole-rock geochemistry to provide an understanding of the petrogenesis of the hornfels close to the contact with the Stillwater Complex. The rocks investigated are predominantly quartz-plagioclase-cordieriteorthopyroxene- biotite ± spinel hornfels, Fe-Cu-Ni sulfides, and Fe-Ti oxides. One sample contains garnet (Alm73-77Pyr20-17Sps3Grs4) in addition to cordierite. Three samples contain cordierite-hercynite symplectites surrounded by a narrow mantle devoid of orthopyroxene, plagioclase, and biotite that are roughly ovoid to lenticular in shape. Partially melted samples show back reaction between crystallizing melt and restite such that retrograde skeletal intergrowths of biotite and quartz in the mesosome were developed. Average T-P conditions of hornfels and melt genesis are 786 °C and 3.7 kbar. Back reactions that produced biotite occurred at somewhat lower temperatures. Whole-rock geochemistry suggests that the protolith of the hornfelsic rocks was likely a mixture of greywacke and additional intermediate to mafic volcanogenic components.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
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- Metamorphosed Ordovician Fe- and Mn-rich rocks in south-central Maine: From peri-Gondwanan deposition through Acadian metamorphism
- Boralsilite, Al16B6Si2O37, and “boron-mullite:” Compositional variations and associated phases in experiment and nature
- Prograde muscovite-rich pseudomorphs as indicators of conditions during metamorphism: An example from NW Maine
- Wagnerite in a cordierite-gedrite gneiss: Witness of long-term fluid-rock interaction in the continental crust (Ile d’Yeu, Armorican Massif, France)
- Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions
- Multiple titanium substitutions in biotites from high-grade metapelitic xenoliths (Euganean Hills, Italy): Complete crystal chemistry and appraisal of petrologic control
- Proto-polymorphs of jimthompsonite and chesterite in contact-metamorphosed serpentinites from Japan
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- Single-crystal 40Ar/39Ar age variation in muscovite of the Gassetts Schist and associated gneiss, Vermont Appalachians
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- Crystal chemistry of phlogopite from Vulture-S. Michele Subsynthem volcanic rocks (Mt. Vulture, Italy) and volcanological implications
- Beneath the Stillwater Complex: Petrology and geochemistry of quartz-plagioclasecordierite (or garnet)-orthopyroxene-biotite ± spinel hornfels, Mountain View area, Montana
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- Vibrational properties of δ-AlOOH under pressure
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- Rhönite in Luna 24 pyroxenes: First find from the Moon, and implications for volatiles in planetary magmas
- New high-pressure B2 phase of FeS above 180 GPa
- Magnesium K-edge EXAFS study of bond-length behavior in synthetic pyrope-grossular garnet solid solutions
