Potassium-Argon Dates of Biotites from Cordilleran Granites

Abstract

Biotites from 20 plutons of the Cordillera of North America, principally from British Columbia, have been dated using the potassium-argon method. Samples were selected from well-known intrusive masses in an attempt to compare physically measured dates with relative age established by geological mapping; most dates confirm the geological age interpretations. The Cordillera underwent granitic intrusion or orogeny at five times separated by long periods of quiescence. The earliest orogeny, of Devonian (Acadian) age, occurred 350-360 million years ago and resulted in intrusion of plutons as widely separated as the Ice River syenite of the Rocky Mountains and the Fitton granite of the northern Yukon. Early Mesozoic orogeny resulted in the emplacement of the upper Triassic or lower Jurassic Guichon (186 m.y.) and Topley (163 m.y.) batholiths of central British Columbia. The major Cordilleran intrusion occurred 95–100 million years ago, in about the middle of the Cretaceous, with emplacement of the Coast Range batholith and early phases of the Nelson batholith of British Columbia, and the Cassiar and Itsi batholiths of Yukon. In late Cretaceous time, 80 million years ago, the Bayonne pluton, British Columbia, and Boulder batholith and Marysville stock of Montana were emplaced. During Rocky Mountain orogeny (Eocene), 50–60 million years ago, late phases of the Nelson batholith and a number of plutons such as Coryell were emplaced. Eighteen million years ago, in the Miocene, small granitic bodies intruded the Cascade Range; these are believed to be the youngest exposed batholithic rocks in North America.