Abstract
The Mount Helen Structural Belt, located in the Wind River Mountains, Wyoming, formed during high temperature ductile deformation at about 2.7 to 2.8 Ga. Rock types within the structural belt are dominated by mylonitic orthogneiss which anastomoses around boudins of relatively undeformed orthogneiss. Changes in petrology, chemistry, and fluid inclusions were studied at orthogneiss — mylonitic gneiss transitions. Deformation occurred at approximately 7000 C and 4.5 to 5.0 kb. At these P-T conditions the dominant petrological change was physical grain size reduction by ductile processes. No major mineral reactions accompanied shearing and the mineralogy of both orthogneiss and mylonitic gneiss remained basically unchanged. Fluid inclusions within the mylonitic gneiss contain saline brines while inclusions in the relatively undeformed orthogneiss are CO2 rich. No systematic major element redistribution can be attributed to shearing in the Mt. Helen Structural Belt. Local modification of trace element chemistry is related to minor changes in accessory mineralogy related to the formation of sphene. High temperature ductile deformation in the presence of aqueous brines is not a sufficient mechanism to modify the chemistry of the deep crust when major mineral reactions are absent.
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© 1989 Kluwer Academic Publishers
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Hulsebosch, T.P., Frost, B.R. (1989). Mineral Changes, Element Mobility, and Fluids Associated with Deep Shearing in the Mount Helen Structural Belt, Wyoming, USA. In: Bridgwater, D. (eds) Fluid Movements — Element Transport and the Composition of the Deep Crust. NATO ASI Series, vol 281. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0991-5_14
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DOI: https://doi.org/10.1007/978-94-009-0991-5_14
Publisher Name: Springer, Dordrecht
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