ABSTRACT

Potassium-argon dates for a number of volcanic and plutonic igneous rocks from the North American Cordillera suggest that the principal Mesozoic intrusives may be late Cretaceous in age (Laramide), rather than Jurassic (Nevadan).

Sanidine from the Crowsnest volcanies, Alberta, of Mid-Cretaceous age, gives a potassium-argon age of 96 million years. Two samples of granitic rocks and a feldspar phenocryst from the Sierra Nevada batholiths, California, gives ages of 88, 72 and 68 m.y., similar to a 71 m.y. age obtained for orthoclase from the Laramide Boulder batholith, Montana.

A study of heavy mineral residues from the Cretaceous sedimentary section and comparison with those from the Cordilleran intrusive suggests, however, that during lower Cretaceous time granitic intrusives were emplaced and unroofed in the Cordilleran area. The 105-m.y. lead alpha-activity age of zircon separated from the Nelson batholith, British Columbia, indicates that it may have been one of these early plutons.

(J. H. Reynolds and J. Lipson, of the Department of Physics, University of California, Berkeley, contributed the potassium-argon age determinations reported in this paper.)

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