Abstract

Radiometric ages have been measured on rocks of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths (St. Kevin Granite and granite of Cross Creek), and various minor intrusive rocks.

A whole-rock Rb-Sr isochron age on the St. Kevin Granite establishes it as 1390 ± 60 m.y. old. Mineral ages on the St. Kevin and numerous other rocks are either about the same as the St. Kevin 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 St. Kevin Granite yield mica ages within analytical error of the St. Kevin 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 St. Kevin 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 St. Kevin 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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