MAGMATIC EVOLUTION AND THE ORIGIN OF THE DALY GAP, SANTA BARBARA VOLCANO, TERCEIRA ISLAND, AZORES

The origin of compositional gaps (e.g. the Daly Gap) between felsic and mafic volcanic deposits has remained a petrogenetic enigma. Bimodal volcanism at Santa Barbara volcano (Terceira island, Azores) exhibits a distinct Daly gap from 55 to 65 wt.% SiO₂ between basaltic and trachytic magmas. However, we show that the compositional gap is partially filled if hypabyssal cognate xenoliths (herein referred to as syenite nodules) contained in trachytic pumice fall deposits from Santa Barbara are considered. This study integrates petrographic observations, whole-rock major and trace element modeling, and isotope geochemistry with thermodynamic (MELTS) modeling of syenite nodules and trachyte pumice along with whole-rock analyses of basalts from literature. The aim of this study is to determine 1) the petrogenetic relationship between the syenitic nodules and the mafic and felsic volcanic rocks at Santa Barbara; 2) whether syenite nodules represent cumulate or unerupted liquid compositions and 3) the possible origin of the Daly gap at Santa Barbara. 

Preliminary MELTS modeling shows that peralkaline trachytes formed as a result of 80%, low-pressure (1 kbar) fractional crystallization of an assemblage of plagioclase, clinopyroxene, olivine, ilmenite, and apatite from a dry (0.5 wt.%H2O) basaltic magma at oxygen fugacities between FMQ and FMQ-1.

Compatible versus incompatible trace element concentrations, isotopic data, and MELTS modeling are consistent with the basalts, syenitic nodules and trachytes being cogenetic, following a common evolutionary trend by fractional crystallization from a common parental magma. However, petrographic observations and plots of trace element ratios illustrate the role of other magmatic processes during formation of the syenites. Some of the syenite nodules contain amphibole and biotite with feldspars exhibiting dueteric textures, indicating that crystallization took place in the presence of aqueous fluids. These altered syenite nodules also display low K/Rb ratios that diverge from the basalt-trachyte fractional crystallization trend.

Further MELTS modeling suggest that the Daly gap at Santa Barbara may be the result of an abrupt increase in differentiation and viscosity at the beginning followed by a rapid decrease in water content at the end of the Daly gap interval.