Open-system magmatic evolution of andesites and basalts from the Salmon Creek volcanics, southwestern Idaho, U.S.A.

The Salmon Creek volcanics (SCV) comprise 31-26-Ma orogenic andesites and mildly alkaline basalts. Detailed petrologic and geochemical studies of two thick, continuous sections of SCV lavas reveal cyclic compositional variations apparently produced by a systematic progression of magmatic processes through time. The earliest phase of SCV activity produced basalts with trace-element and isotopic compositions indicating an asthenospheric source enriched in subduction-related components. Following eruption of these basalts, an andesitic magma chamber was established within the continental crust. Near the base of the thickest andesite section, compositions of the lavas become more mafic upsection, indicating high rates of magma recharge relative to crystallization. Wall-rock assimilation was most significant in the earliest stages of magma chamber initiation and growth, as shown by the sharp increase in alkalies and covariant changes in Sr, Pb and Nd isotopes in the lowermost andesite lavas. Further upsection, compositional variations in the andesites are more consistent with fractional crystallization as the predominant magmatic process. Observed phenocryst assemblages in the andesites and mass-balance calculations indicate that crystallization was dominated by kaersutitic hornblende with relatively little plagioclase. Hornblende-dominated assemblages suggest that the magmas were hydrous (≥2% H₂O in the melt) and evolved at moderate pressures in the lower to middle crust (3-15 kbar).