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

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.