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Age of the basement rocks of southwest Montana.

Geological Society of America Bulletin

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Abstract

Rb-Sr analyses of a suite of quartzofeldspathic gneisses that are interlayered with beds of marble, quartzite, and amphibolite in the Ruby and Tobacca Root Ranges and the Gallatin River canyon of southwest Montana show that the age of metamorphism of these strata occurred about 2750 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 Dillion 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 2762+- 113 m.y. Inclusion of other published data for the Tobacco Root Range yields a best-fit value of 2730+- 85 m.y. This age corresponds closely to that of the principal metamorphic-plutonic epoch of the Beartooth Mountains, to which the term 'Beartooth orogeny' has been applied. It also demonstrates that the major Precambrian metasedimentary sequences of the region are of Archean age. - Authors

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Age of the basement rocks of southwest Montana.
Series title:
Geological Society of America Bulletin
Volume
91
Issue:
1 pt1
Year Published:
1980
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geological Society of America Bulletin
First page:
11
Last page:
15
Number of Pages:
5