Geochronology of the St. Kevin granite and neighboring precambrian rocks, northern Sawatch Range, Colorado

Geological Society of America Bulletin
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Radiometric ages have been measured on rocks of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths (StKevin Granite and granite of Cross Creek), and various minor intrusive rocks. A whole-rock Rb-Sr isochron age on the StKevin Granite establishes it as 1390 ± 60 m.y. old. Mineral ages on the StKevin and numerous other rocks are either about the same as the StKevin whole-rock age or younger by as much as 200 m.y., even where the relative age is known to be older. Some minor Precambrian intrusive masses that are probably younger than StKevin Granite yield mica ages within analytical error of the StKevin age, indicating that these rocks can be younger than the granite by only a few tens of millions of years. The mica ages, both Rb-Sr and K-Ar, are thought to be minimal, but a K-Ar age of 2020 m.y. on horn-blende probably reflects excess argon. Mica ages from all rocks known geologically to be older than StKevin Granite are low and are interpreted as heating ages reflecting intrusion of the granite, in some cases modified further by heating during Laramide time. In this area, Precambrian intrusion and deformation had largely ended by 1200 or 1300 m.y. ago. Plutonism, represented here by the StKevin Granite and elsewhere by the Silver Plume and other granites, probably accounts for the numerous mineral ages of about 1300 m.y. previously reported from Colorado although weak regional metamorphism may also have been a factor. 

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Publication type Article
Publication Subtype Journal Article
Title Geochronology of the St. Kevin granite and neighboring precambrian rocks, northern Sawatch Range, Colorado
Series title Geological Society of America Bulletin
DOI 10.1130/0016-7606(1966)77[1109:GOTSKG]2.0.CO;2
Volume 77
Issue 10
Year Published 1966
Language English
Publisher Geological Society of America
Description 12 p.
First page 1109
Last page 1120
Country United States
State Coloradao
Other Geospatial Sawatch Range
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