C. E. Hedge
H.H. Thomas
T.W. Stern
R. C. Pearson
1966
<p><span>Radiometric ages have been measured on </span>rocks<span> of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths (</span>St<span>. </span>Kevin<span> </span>Granite<span> and </span>granite<span> of Cross Creek), and various minor intrusive </span>rocks<span>. A whole-</span>rock<span> Rb-Sr isochron age on the </span>St<span>. </span>Kevin<span> </span>Granite<span> establishes it as 1390 ± 60 m.y. old. Mineral ages on the </span>St<span>. </span>Kevin<span> and numerous other </span>rocks<span> are either about the same as the </span>St<span>. </span>Kevin<span> whole-</span>rock<span> age or younger by as much as 200 m.y., even where the relative age is known to be older. Some minor </span>Precambrian<span> intrusive masses that are probably younger than </span>St<span>. </span>Kevin<span> </span>Granite<span> yield mica ages within analytical error of the </span>St<span>. </span>Kevin<span> age, indicating that these </span>rocks<span> can be younger than the </span>granite<span> 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 </span>rocks<span> known geologically to be older than </span>St<span>. </span>Kevin<span> </span>Granite<span> are low and are interpreted as heating ages reflecting intrusion of the </span>granite<span>, in some cases modified further by heating during Laramide time. In this area, </span>Precambrian<span> intrusion and deformation had largely ended by 1200 or 1300 m.y. ago. Plutonism, represented here by the </span>St<span>. </span>Kevin<span> </span>Granite<span> and elsewhere by the Silver Plume and other granites, probably accounts for the numerous mineral ages of about 1300 m.y. previously reported from </span>Colorado<span> although weak regional metamorphism may also have been a factor. </span></p>
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10.1130/0016-7606(1966)77[1109:GOTSKG]2.0.CO;2
en
Geological Society of America
Geochronology of the St. Kevin granite and neighboring precambrian rocks, northern Sawatch Range, Colorado
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