Potassic Mafic Lavas of the Bearpaw Mountains, Montana: Mineralogy, Chemistry, and Origin

Abstract

The volcanic rocks of the Bearpaw Mountains are part of the Montana high-potassium province, emplaced through Archaean rocks of the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks, dominated by minettes and latites, have a volume of U 825 km3. The minettes range in composition from 20 to 6% MgO. The more magnesian varieties contain the phenocryst assemblage forsterite + Cr-spinel + diopside ± phlogopite. More evolved rocks are olivine-free, with an assemblage of either salite + phlogopite + pseudoleucite or salite + phlogopite + analcime. The analcime is thought to be secondary after leucite, produced by loss of potassium from the minettes. Mineral chemistry and textures, especially of clinopyroxenes, indicate that mixing between minette magmas of varying degrees of evolution was commonplace. Compositional variation was further extended by accumulation of olivine + spinel + clinopyroxene phenocrysts, and by the preservation of mantle xenocrysts in the minettes. The primary minette magmas are inferred to have had 12–14% MgO and to have been generated at 30 kb from an olivine + diopside + phlogopite-bearing source. The primary magmas evolved dominantly by fractionation of olivine + diopside. The minettes have high contents of large ion lithophile elements (LILE) and light rare earth elements (LREE), with K2O up to 6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0 ranges from 0.707 to 0.710 and £Nd varies from –10 to –16. These data, plus high LILE/HFSE (high field strength elements) values, are interpreted to show that the minettes contain at least three different mantle components. The lithosphere was initially depleted in Archaean times, but was metasomatically enriched in the Proterozoic and in the late Cretaceous and early Tertiary. The latites have many chemical features in common with the minettes, such as potassic character and high LILE/HFSE values. They formed by fractional crystallization of minette magma in combination with assimilation of crustal rocks; this process enriched the magmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemical data for phenocrysts and bulk rocks in minettes suggest mixing between minette and latite magmas.