| A quantitative understanding of trace-element distribution between mineral and liquid phases during crystallization and melting is prerequisite to developing chemical differentiation models for describing magmatic evolution. The petrogenesis of highly evolved silicic rocks is especially problematic, given the many differentiation mechanisms postulated for their formation, including, but not limited to: partial melting of intermediate-composition crustal rocks; protracted crystal fractionation of mafic magma, which itself may have a complex origin involving partial melting of ultramafic-composition mantle rocks followed by fractional crystallization, assimilation, and/or magma mixing; crystal fractionation of mafic magma coupled—or decoupled—with assimilation of crustal rocks, magma mixing of mafic and felsic magmas followed by crystal fractionation; and any combination of these or other (e.g., volatile transfer, thermogravitational diffusion, liquid immiscibility, etc.) processes.; This dissertation investigates the application of trace-element geochemistry towards understanding the petrogenesis of a specific class of felsic rocks, peralkalic silicic rocks (peralkalic quartz trachyte, comendite, and pantellerite), along with coeval high-alkali metaluminous rocks (metaluminous quartz trachyte and rhyolite). Peralkalic silicic rocks may represent the end-result of protracted crystal fractionation of transitional to alkalic basalt, whereas coeval metaluminous felsic rocks may represent magmas that have assimilated significant amounts of Al-rich crustal rocks, coupled with crystal fractionation. Incompatible trace-element (Rb, Y, Zr, Nb, and REE) partition coefficients for alkali feldspar/peralkalic silicic magma vary strongly with whole-rock silica content and agpaitic index (mol Na + K/Al) in these systems and these parameters can be used to estimate partition coefficient values used in modeling fractional crystallization processes. Partition coefficients for compatible elements (Sr and Ba), however, demonstrate largely non-systematic behavior due to either kinetics, post-crystallization alteration, and/or high analytical error at low detection limits for these elements that may be strongly depleted in silicic magmas, thus minimizing their utility. Equations based on multivariate linear regression analysis of trace-element partition coefficients and melt and crystal chemical parameters are presented for several trace elements; these will help future workers better constrain their choice of trace-element partition coefficients when modeling crystal fractionation in peralkalic silicic systems. |
