Genetic relations of shoshonitic and absarokitic magmas, Absaroka Mountains, Wyoming

The nature, origin, and tectonic significance of shoshonitic volcanism is currently the subject of widely differing views. In the type locality in the Absaroka Range, the rocks consist of a diverse group of lavas, all of Mid-Eocene age. High in the volcanic pile are subordinate volumes of absarokite and shoshonite flows, both of which contain calcic plagioclase and sanidine coexisting in the groundmass. Shoshonites contain plagioclase, olivine, and pyroxene phenocrysts; absarokites contain only olivine and pyroxene phenocrysts. A few absarokites contain modal leucite. A chemical study was made of six shoshonites and three absarokites from this area.Although resorption and zoning indicate disequilibrium in both types of rocks, there is no geochemical evidence of magmatic contamination. Calculated crystal/liquid distribution coefficients are in close agreement with previously determined values for basalts. Decrease in pressure during ascent is a likely cause for the observed disequilibrium.Mass balance calculations show that it would be possible to form a shoshonite by fractionation of olivine and pyroxene from an absarokitic magma. Trace element abundances are consistent with this relationship. Petrographic evidence suggests, however, that fractionation of plagioclase played a role in the formation of shoshonites.A high-pressure origin is consistent with rare earth compositions, and a high magmatic temperature is indicated by the composition of the plagioclase. A calculation of ln a_K^liquid2O for a reaction involving eclogite and a reaction involving a tested rock (Marsh and Carmichael, 1974) suggests a high-pressure origin for a latitude underlying the shoshonites and absarokites. This calculation is subject to large variances because of its sensitivity to estimated equilibrium temperatures.Comparison with lavas of similar composition indicates that shoshonites and absarokites of the Absaroka Range are influenced by their continental setting. There is little evidence for the generation of these magmas in a subduction environment.