BETWEEN LIFE AND DEATH: GEOCHEMICAL MATURATION OF THE WYOMING CRATON

A fundamental change in the continental crust took place by the end of the Archean: older crust composed of TTG and mafic rocks transitioned to a more K-rich granitic crust. Examination of Meso- and Neoarchean rocks in the northern Laramie Mountains suggests that in the Wyoming province this transition was complete by between 3.0 and 2.75 Ga.

The study site exposes a suite of multiply-folded mafic and ultramafic rocks, tonalite, quartzite, sillimanite-bearing metapelitic schist, and granitic gneisses. The granitic gneisses are the most extensive units, and include biotite granite, leucogranite, and garnet granite gneiss. These rocks form the roof of the 2.62 Ga Wyoming batholith, which occupies most of the northern and central Laramie Mountains.

U-Pb zircon geochronology on detrital zircons from sillimanite schist are 3.3-3.4 Ga and from quartzite are 3.3-3.6 Ga, indicating a Paleoarchean protolith. The granitic gneisses range in age from 2.75 to 3.0 Ga. They are ferroan and calc-alkalic, are LREE-enriched with strongly negative Eu anomalies, and have higher K₂O/Na₂O than the Neoarchean granitic batholiths in the Wind River and Bighorn Mountains. The garnet granite gneiss is strongly peraluminous (ASI > 1.1). The granitic gneisses are associated with biotite schists and sillimanite schists that have REE patterns similar to post-Archean shale. Sm-Nd isotopic data indicate that whereas the tonalitic and mafic rocks are relatively juvenile, the metasedimentary rocks and granitic gneisses incorporate older crustal sources.

The 3.0-2.75 Ga granitic gneisses and associated metasedimentary rocks of the northern Laramie Mountains fill an important gap in the Archean history of the south-central Wyoming province between 3.3 Ga TTG crust exposed in the Granite Mountains and voluminous 2.63-2.62 Ga calc-alkalic magmatic arc rocks in the Laramie, Granite, and Wind River Mountains. Globally, strongly peraluminous granites are rare prior to the Neoarchean. The presence of garnet granite gneiss and aluminous metasedimentary rocks in the northern Laramie Mountains indicate that the secular change from TTG to evolved crust occurred well before the Archean-Proterozoic boundary in the Wyoming province.