Sunlight volcano is a dissected, but well-preserved eruptive center in the eastern, high-K belt of the Absaroka volcanic province. On the basis of high-precision 40Ar/39Ar ages, magmatism at the volcano commenced prior to 49.6 Ma and continued until ca. 48.1 Ma. Stratigraphic relationships along with geochemical and 40Ar/39Ar data distinguish three principal stages of effusive volcanism that are interpreted to reflect the inception, climax, and decay of a crustal magmatic system related to three major pulses of basalt and high eruptive rates. During the first two stages, shoshonitic and latitic lavas with small but distinct compositional ranges accumulated. The third stage included eruption of picritic basaltic lavas and intrusion and solidification of magmas in the volcanic edifice. As this stage produced magmas compositionally more evolved and diverse (basalts to trachytes) than in previous stages, it likely represents the terminal pulse of basalt to the system, followed by cooling, protracted differentiation, and cessation of eruptive activity.

Petrographic, geochemical, and Sr and Nd isotope data for Sunlight igneous rocks indicate that crystallization differentiation was the dominant magma-evolution process, although phenocryst accumulation and crustal contamination also occurred. Evidence for phenocryst accumulation is most pronounced in final-stage rocks. This relationship is interpreted to reflect recharge of late magmas into mature chambers laden with crystals deposited during earlier stages. The importance of crystallization differentiation at Sunlight volcano contrasts with recent petrogenetic models for calc-alkaline magmas in the Absaroka volcanic province, which require a more important role for early magma mixing. The results presented here indicate that across-strike increases in K2O contents of rocks in the field derive from variations in the conditions of crustal differentiation, rather than from mantle processes involving a subducting slab.

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