Precambrian geochronology and geology of the Boxelder Canyon area, northern Laramie Range, Wyoming

Abstract

The Laramie granite underlies most of the northern half of the Laramie Range and forms a sharply discordant, nearly horizontal contact with older metamorphic rocks west of Boxelder Creek. Seven whole-rock samples of Laramie granite yield a Rb-Sr age of 2,567 ± 25 m.y., with an initial Sr87/Sr86 ratio of 0.7026. The metamorphic rocks consist mainly of gray granite gneiss (2,759 ±52 m.y. old) and leucogranite (2,776 ± 35 m.y. old), with sillimanite-bearing migmatitic gneiss (3,020 ± 221 m.y. old), amphibolite, and ultramafic rocks occurring interlayered with or as pods and stringers in the granite gneiss and leucogranite. The metamorphic rocks form a large fold with a nearly vertical axis. The granite gneiss and leucogranite are inferred to have been intruded magmatically into sillimanite-bearing gneiss, amphibolite, and ultramafic rocks between 2,700 and 2,800 m.y. ago. A low initial Sr87/Sr86 ratio for the granite gneiss suggests an origin in the lower crust or upper mantle. An unusually high initial Sr87/Sr86 ratio for the leucogranite suggests derivation largely from an older crustal source with a high Rb/Sr ratio.