Foster, C. T. and Reagan, M. K. and Kennedy, S. G. and Smith, G. A. and White, C. A. and Eiler, J. E. and Rougvie, J. R. (1999) Insights into the Proterozoic geology of the Park Range, Colorado. Rocky Mountain Geology, 34 (1). pp. 7-20. ISSN 1555-7332 http://resolver.caltech.edu/CaltechAUTHORS:20121106-152401438
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Proterozoic rocks in the Park Range comprise an assemblage of highly deformed metavolcanic and metasedimentary supracrustal rocks intruded by numerous plutons. The oldest rocks in the region are a suite of bimodal metavolcanic and associated metasedimentary rocks on Farwell Mountain. These rocks are similar to the Green Mountain Formation to the north, and are geochemically similar to modern continental volcanic arcs related to subduction. The most widely exposed Proterozoic supracrustal unit in the Park Range consists of a highly metamorphosed and deformed, bimodal volcanic sequence with trace-element patterns similar to those of continental interior bimodal suites instead of subduction-related volcanism. The metavolcanic rocks are overlain by a suite of metasedimentary rocks. Protoliths for the basal metasedimentary rocks are graded sequences of feldspar-rich lithic sandstones and conglomerates interpreted as turbidites. These grade stratigraphically upward into a sequence of interbedded shales and finegrained quartz arenites, similar to those found on passive-margin shelves. The top of the sedimentary sequence consists of a medium- to coarse-grained, cross-bedded quartzite. The supracrustal rocks are intruded by plutonic rocks with compositions from gabbro to granite and ages of 1.78 to 1.735 Ga. The youngest Proterozoic unit in the area is the ∼1.4-Ga Mount Ethel pluton. Upper amphibolite-facies regional metamorphism produced abundant sillimanite in aluminous rocks throughout the Park Range and overprints structural fabrics associated with the older intrusions. Garnet-biotite/GASP thermobarometry indicates temperatures for this event ranging from 550–700°C and pressures of 4 to 6 kbar for most of the region south and east of greenschist-facies rocks at Farwell Mountain. Evidence of an earlier period of higher pressure metamorphism is present near Lester Mountain where kyanite that contains small blebs of staurolite has partly broken down to sillimanite. Evidence for a late, lower-pressure rehydration event is also recorded at Lester Mountain. The dominant fabric in the supracrustal rocks throughout the range consists of a steeply dipping northeast-southwest-striking foliation with a steeply plunging stretching lineation. This fabric is also recorded in the oldest of the felsic plutonic rocks, although the amount of strain in the dated plutonic rocks decreases with age. The intensely deformed Soda Creek-Fish Creek shear zone might be a suture between major terranes in the Park Range. There are also changes in metamorphic grade and ages of plutonic rocks that may be related to the shear zone. The main deformation along this shear zone predated the Mount Ethel pluton and was northside-down. However, some left-lateral displacements along late mylonite zones offset rocks attributed to the Mount Ethel magmatic system. A second candidate for a suture is the area between Farwell Mountain and Lester Mountain, where metamorphic grades and compositions of metavolcanic rocks change abruptly.
|Additional Information:||© 1998 University of Wyoming. Manuscript submitted December 30, 1997; Revised manuscript submitted February 24, 1998; Manuscript accepted May 8, 1998. The authors wish to acknowledge the prompt and helpful reviews of a previous version of this manuscript by Jeff Noblett, Colin Shaw, and Karl Karlstrom. Kevin Chamberlain made a number of very useful comments on a preliminary version of the manuscript and also provided insights into the geochronology and tectonics of southern Wyoming and northern Colorado. The late George Snyder suggested the Park Range for study, led several field trips to the area, and provided perceptive advice on numerous occasions. Ian Steele facilitated microprobe analyses on the Cameca SX-50 at the University of Chicago. Analyses and field work were partially funded by the University of Iowa Foundation, UNOCAL, and MOLYCORP.|
|Subject Keywords:||Proterozoic stratigraphy, Proterozoic volcanism, Proterozoic metamorphism, Proterozoic shear zones, Proterozoic sutures, Park Range, Colorado|
|Official Citation:||C. T. Foster, M. K. Reagan, S. G. Kennedy, G. A. Smith, C. A. White, J. E. Eiler, and J. R. Rougvie Insights into the Proterozoic geology of the Park Range, Colorado Rocky Mountain Geology, Spring, 1999, v. 34, p. 7-20, doi:10.2113/34.1.7|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||08 Nov 2012 19:35|
|Last Modified:||08 Nov 2012 19:35|
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