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Metamorphism and deformation near the 1.4-Ga Mount Ethel pluton, Park Range, Colorado

Department of Geology, University of Iowa, Iowa City, Iowa 52242, U.S.A.

The 1.4-Ga Mount Ethel pluton is a northeast-trending elliptical body of granodiorite to quartz monzonite surrounded by Lower Proterozoic (1.8–1.7 Ga) rocks in the Park Range, northeast of Steamboat Springs, Colorado. The contact along the northern and southern margins of the pluton is concordant with the northeast-striking regional foliation, whereas the western margin is discordant to the regional foliation. The southern margin of the pluton is adjacent to the subvertical, northeast-striking, northwest-dipping Soda Creek–Fish Creek shear zone, which is located entirely within metasedimentary and metavolcanic country rocks. Kinematic indicators within the shear zone yield a north-side-down sense of shear with a left-lateral component. In contrast to several other well-studied 1.4-Ga plutons associated with shear zones in the western U.S., penetrative strain related to the shear zone does not appear to extend into the pluton. Late dikes, which appear to be genetically related to the Mount Ethel pluton, cut the main penetrative fabric of the shear zone but are also offset by narrow, discontinuous mylonites of the shear zone. Scarce discontinuous mylonites are also present along the southern margin of the pluton. The pluton contains magmatic fabrics that parallel the strike of the shear zone along the north and south margins of the pluton, but are at a high angle to it along the western margin. These data suggest that early deformation associated with the Soda Creek-Fish Creek shear zone occurred prior to pluton emplacement, that the Mount Ethel pluton was emplaced along a northeast-striking zone of anisotropy, and that some shearing also took place after crystallization and cooling of the pluton.

Country rocks in the vicinity of the Mount Ethel pluton with garnet-biotite-plagioclasemuscovite ± sillimanite have two generations of garnet: one that is syntectonic and another that is post-tectonic relative to the northeast-striking foliation. Core compositions of pre/syntectonic garnet paired with matrix biotite compositions yield temperatures of > 600°C at 5 kbar that are associated with early (pre-Mount Ethel) deformation within the Soda Creek-Fish Creek shear zone. Core compositions of post-tectonic garnet paired with matrix biotite compositions reveal a temperature gradient on the south side of the pluton that increases from 540°C 5 km away to > 630°C within 1 km of the Mount Ethel pluton. This may represent the thermal aureole around the intrusion, or it may represent a gradient that pre-dated the pluton. Garnet-rim compositions from both syntectonic and post-tectonic garnet, paired with compositions of biotite near garnet, yield consistent temperatures of 550 ± 50°C that are spatially unrelated to the Mount Ethel pluton. These rim temperatures may be related to a third thermal event; or alternatively, they may represent partial re-equilibration of garnet rims with nearby biotite during cooling following the last metamorphism.

Key Words: Soda Creek-Fish Creek shear zone • Mount Ethel pluton • 1.4-Ga plutonism • Proterozoic deformation • Proterozoic metamorphism

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