PETROGENSIS OF SHOSHONITIC MAGMAS AT THE SUNLIGHT VOLCANO, ABSAROKA VOLCANIC PROVINCE, WYOMING

The Sunlight volcano (49.6-48.1 Ma) in the Absaroka Volcanic Province (AVP) is the type locality for shoshonitic magmas worldwide. Field, ⁴⁰Ar/³⁹Ar, and geochemical data indicate at least three stages in the petrologic evolution of the volcano. The first stage is characterized by shoshonitic to latitic lava flows of the Jim Mountain Member. These flows are distinguished by modestly increasing K₂O (2.1 to 3.6 wt%) with increasing SiO₂ (51.7 to 56.7 wt%). The second stage is characterized by magmas more potassic than previous eruptions. Flows of the lower Trout Peak Trachyandesite are shoshonitic to latitic with 52.7 to 55.4 wt% SiO₂ and 3.0 to 4.7 wt% K₂O. The third stage is represented by olivine-rich potassic trachybasalts and shoshonites of the upper Trout Peak Trachyandesite. These flows have 49.6 to 51.5 wt% SiO₂ and continue the trend of increasing K₂O contents upsection (1.8 to 3.1 wt%). The waning stage of magmatism is represented by the shoshonitic to trachytic Copper Lakes intrusion, which is evolved internally (52.3 to 64.1 wt% SiO₂) and relative to previous volcanic units.

Many chemical features of the rocks, such as non-linear trends on variation diagrams and REE patterns that become increasingly concave upward with SiO₂, can be modeled by fractionation of clinopyroxene- or amphibole-rich phenocryst assemblages. Other trace element and isotopic data require variable mantle sources and assimilation of continental crust. These data indicate that mantle sources for the magmas became more radiogenic in both ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios with time and that the crust assimilated had low Sr and high Nd isotopic ratios relative to parental magmas.

The importance of fractionation during differentiation of Sunlight magmas contrasts with the results of models developed for calc-alkaline magmas in the AVP, which require a more important role for magma mixing. These results suggest that regional K₂O trends in the AVP may not reflect distinct mantle compositions or melting processes, but instead may derive from variations in the conditions of crustal differentiation.