REMOTE SENSING CHARACTERIZATION OF A MAJOR PRE-SUPERERUPTION EFFUSIVE UNIT IN THE BLACK MOUNTAINS OF NORTHWEST ARIZONA

The Peach Spring Tuff (PST) in the southern Black Mountains of western Arizona was produced 18.8 Ma during a massive, caldera forming supereruption. Stratigraphically underlying the PST is a ~ 1km- thick sequence of ~19 Ma, phenocryst-rich (~25-35% plagioclase, and biotite, ± minor pyroxene and hornblende) trachytic lavas (mostly 62-65 wt% SiO₂, >4wt% K₂O) with interbedded sediments. Previous field studies estimated the volume of this pre-PST unit (~10³ km³) and characterized the lavas as compositionally homogeneous, but their full extent, volume, and geochemical variability are not fully known; further characterization of this unit is hampered by its wide distribution across a remote and rugged region of the southern Black Mountains. Since spatial and temporal proximity of the trachyte lavas and PST source caldera suggest they may be petrogenetically related, understanding the extent and compositional diversity of this pre-PST unit is essential for reconstructing how the PST magma body formed.

To this end, we used Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery to identify and differentiate pre-PST trachytes in the region. A 7-3-1 RGB band combination was selected using Environment for Visualizing Images (ENVI) software to highlight the trachyte, and suggested some possible compositional variations. Image collected spectra, however, vary only slightly across the trachyte with characteristic absorption features between 0.6 and 0.7 μm as well as at 2.1 and 2.4 μm; based on the observed phenocryst assemblage within this unit, these absorptions likely correspond with biotite and plagioclase, respectively.

Overall geochemical homogeneity is supported by the almost uniform nature of spectra across the unit; minor variations visible in the imagery may be caused by factors other than geochemistry, such as weathering and topography. Despite the generally monotonous composition, high resolution imagery and field observations show at least part of this unit is divisible into flows separated by sedimentary packages (which appear to be spectrally and mineralogically similar to the trachytes). These distinct flows, in addition to rare outliers in new geochemical analysis (56-71 wt% SiO₂) and in spectral data, reveal the complexity of this unit and suggest further avenues of study.