Two-Stage AFC-FC Model for the Evolution of Large-Volume Rhyolitic Ignimbrites (LRIs).

Questions concerning the generation of large-volume rhyolitic ignimbrites (LRIs) involve the relative roles of fractional crystallization (FC), crustal assimilation (A) and magma source. Major and trace element data, combined with petrologic observations can be used to assess the role of melting and the order and extent of fractional crystallization. Isotopic data can be used to evaluate the role of magma source versus crustal assimilation. Many LRIs (e.g. Mount Taylor, NM; Long Valley, CA; Chalupas, Ecuador) show evidence for multiple stages of evolution within the crust with the amount of assimilation varying as magma evolves from basalt to rhyolite. We evaluate the processes involved using new data from the Clear Lake, CA, and Chalupas, Ecuador, volcanic suites. A key observation is that isotopic variability occurs only in lavas with < 57% SiO₂, with very little difference in the Nd or Sr isotopes apparent in lavas with greater than 57% SiO₂. Hence, compositions ranging from andesite through rhyolite can all be traced to the same magma source and parental magma composition. We have modeled the evolution of these magmatic systems with a two-stage finite difference model that accounts for major and trace elements as well as isotopes. The fractionating mineral assemblage is determined by petrographic observations and varies as the magma evolves. Distribution coefficients depend on the fractionating assemblage and the SiO₂ content of the magma. During Stage I, primitive basaltic magma originating from below the lithosphere evolves to andesite through AFC in the lower crust. Stage I occurs either in small magma chambers or in dikes; assimilation is allowed by the high wall rock temperatures. Once the magma reaches approximately 57% SiO₂ it continues its ascent through the crust and is either erupted as andesite or goes to Stage II evolution in a shallow magma chamber. During stage II, wall rock temperatures are too low to allow for crustal assimilation and crystallization is fast hence andesite evolves to rhyolite through FC. What physical process (eg. sidewall crystallization) is responsible for the FC stage is not constrained. What limits Stage I to magma with less than 56 \- 59% SiO₂ is also unclear (density, viscosity?), but assimilation is required by the large shifts in Nd isotopes observed in some systems (with the caveat that some of this shift may occur in the mantle lithosphere).