VOLUMINOUS INTERMEDIATE, EFFUSIVE MAGMATISM IN THE BLACK MOUNTAINS, AZ, PRECEDING THE PEACH SPRING SUPERERUPTION, AND EVALUATION OF ITS POTENTIAL RELATIONSHIP TO THE SUPERVOLCANO MAGMA CHAMBER

Voluminous intermediate, effusive lavas erupted at ~ 19 Ma in the Southern Black Mountains, AZ within the Colorado River Extensional Corridor. These lavas extend from the southern end of the Black Mountains to Union Pass, in an area of ~1000km³ that surrounds the source caldera of the 18.8 Ma Peach Spring Tuff supereruption. Understanding the relationship between these intermediate lavas and the Peach Spring supereruption may provide insight regarding the storage and eruption of such large magmatic systems.

We employed X-ray fluorescence (XRF) analysis of whole rocks and scanning electron microscope (SEM) investigation of textures and phase compositions to characterize the intermediate lavas and evaluate their possible relationship to the PST. Whole rock elemental compositions of eleven samples representative of the intermediate lavas and SEM analyses of groundmass permitted comparisons among PST pumice data using trace element equilibrium crystal-melt modelling and fractional crystallization modelling.

SiO₂ concentrations of the lavas range from 57-71 wt%, with an average of 63 wt % and K₂O from 4.1 to 5.4 wt%. Most are classified as trachyte in the total alkalis vs. silica classification. Phenocrysts, dominantly plagioclase and biotite, are consistently abundant (25-35%). Groundmass compositions estimated from SEM analysis of two samples suggest melt SiO₂ of about 65 wt %. Our data and modeling appear to refute a simple fractionation relationship between the pre-PST intermediate lavas and the PST, and a much lower Si concentration in the SEM groundmass data using trace element modelling. Both bulk rock analyses and estimated groundmass (~melt) compositions (major elements by SEM, trace elements by modeling) of the lavas are considerably less evolved than PST, and the trace element compositions of PST cannot be matched by fractional crystallization models.