GEOCHEMICAL AND PETROLOGIC INVESTIGATION OF OPEN SYSTEM PROCESSES IN POST- SUPERERUPTION MEADOW CREEK TRACHYTE, SOUTHERN BLACK MOUNTAINS, AZ

The Meadow Creek trachyte lava (MC) erupted 17.58 ± 0.05 Ma (McDowell et al. 2016) from the southern Black Mountains volcanic center in the Colorado River Extensional Corridor of NW Arizona, ~1.2 Ma following the Peach Spring supereruption. It is exposed in Meadow Creek Basin, <10 km east of the Peach Spring caldera. The limited lateral extent of the unit (3 km), exposed thickness (300 m), auto-brecciated margins, and lack of distinct flow units suggest that MC is a dome. The dome formed a topographic high against which younger Sitgreaves Tuff was deposited in buttress unconformity (Schwat et al 2016).

Plagioclase and alkali feldspar megacrysts, from ~0.2-5 cm long, commonly display compositional zoning and sieve- and anti-rapakivi-textured rims that indicate initial growth in environments unrelated to host melt. The assemblage of other phenocrysts and/or xenocrysts is large and likewise marked by complex textures: bio, amph, qtz (strongly resorbed), cpx, sphene (titanite), apatite, zircon, and mt+ilm; sphene and amph in particular have well-defined reaction rims... Although textures and the crystal assemblage clearly reveal mingling of diverse materials, bulk compositions, including samples of separated distinct flow bands, fall in a narrow range (64-67wt% SiO₂ [XRF]), indicating that lavas were mixed on hand sample scale and that banding evident in the field does not mark distinct magmas. High Al in amphibole (SEM-EDS) suggests that amphiboles may have equilibrated at depths of ~15-30 km (Schmidt 1992). Sieve-textured zones in feldspars have apparent extreme ternary compositions (SEM-EDS), relatively high in both Ca and K, possibly reflecting very fine-scale mixtures of plag+afs ± interstitial melt. These zones document disequilibrium induced by rapid transition of T, P, or composition of the host magmatic environment.

Our observations and data indicate mixing of distinct magmas at depth, possibly triggering the MC dome eruption (e.g. Cashman and Blundy 2013). They further support inferences based on zircon and whole-rock Hf isotopic data that MC reflects sample-scale mingling of diverse melt and solid materials, possibly indicating remobilization of plutonic cumulate and entrainment into ascending magma (McDowell et al. 2016).