The origin of composition gaps at South Sister volcano, central Oregon: implications for fractional crystallization processes beneath active calc-alkaline volcanoes

South Sister is an active, calc-alkaline strato-volcano located in west-central Oregon in the high Cascades. South Sister contains lavas that range from basalt (50% SiO2) to rhyolite (75% SiO2). Within this compositional spectrum the lavas are restricted to three distinct populations: basalt/basaltic-andesite, andesite and rhyolite. Compositional, textural and mineralogic criteria indicate that the basaltic-andesites are hybrid mixtures of basalt and andesite magma. This implies that the system is characterized by basalt, andesite and rhyolite magma populations that are separated from one another by composition gaps. Major, minor and trace element modeling suggests that the three magma populations are related to one another primarily through fractional crystallization. We propose a standard fractionation model for generating compositional diversity amongst erupted lavas wherein in situ fractional crystallization occurs within a downward descending solidification front. Extraction and eruption of the interstitial fractionated liquid, now separated from the parental liquid by a composition gap, occurs only after the percentage of crystallization within the solidification front has exceeded the appropriate critical crystallinity value for the given parental magma. We envision a magmatic plumbing system characterized by multiple, independently solidifying bodies of magma. Thus, at any given time, discrete bodies of basalt, andesite and rhyolite magma may exist. Interaction and mixing among these bodies could generate the continuous range of lava compositions that characterize most calc-alkaline volcanic complexes.