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Age of the basement rocks of southwest Montana
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1980
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EngineeringGeomorphologySedimentary GeologyEarth ScienceRegional GeologyGeological DataGeochronologyGeological EngineeringLinear ArrayGeographyGeologyGsa Bulletin 1980SedimentologySouthwest MontanaRock PropertiesStructural GeologyExploration GeologyEarth SciencesRock Mechanics
Research Article| January 01, 1980 Age of the basement rocks of southwest Montana H. L. JAMES; H. L. JAMES 1U.S. Geological Survey, Port Townsend, Washington 98368 Search for other works by this author on: GSW Google Scholar C. E. HEDGE C. E. HEDGE 2U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (1): 11–15. https://doi.org/10.1130/0016-7606(1980)91<11:AOTBRO>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. L. JAMES, C. E. HEDGE; Age of the basement rocks of southwest Montana. GSA Bulletin 1980;; 91 (1): 11–15. doi: https://doi.org/10.1130/0016-7606(1980)91<11:AOTBRO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Rb-Sr analyses of a suite of quartzo-feldspathic gneisses that are interlayered with beds of marble, quartzite, and amphibolite in the Ruby and Tobacco Root Ranges and the Gallatin River canyon of southwest Montana show that the age of metamorphism of these strata occurred about 2,750 m.y. ago. The 13 samples analyzed are from rock units that have in the past been assigned stratigraphically to the Pony Group, Cherry Creek Group, and Dillon Granite Gneiss. Except for two samples of anomalous composition, the data define a linear array on an isochron diagram that has a best-fit value of 2,762 ± 113 m.y. Inclusion of other published data for the Tobacco Root Range yields a best-fit value of 2,730 ± 85 m.y. This age corresponds closely to that of the principal metamorphic-plutonic epoch of the Bear-tooth Mountains, to which the term "Bear-tooth orogeny" has been applied. It also demonstrates that the major Precambrian metasedimentary sequences of the region are of Archean age. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.