CONSTRAINING ERUPTIVE EVENTS PRECEDING THE PEACH SPRING TUFF SUPERERUPTION (MIOCENE, AZ): PETROLOGY AND PETROGENESIS OF ASH FALL TUFFS

To better understand the evolution of Miocene magmatism in the northern Colorado River Extensional Corridor (NCREC), we investigate the petrology of ash fall tuffs stratigraphically adjacent to the Cook Canyon and Peach Springs Tuffs (CCT, PST) and their possible correlatives in the Transition Zone and Colorado Plateau. The 18.8 Ma supereruptive PST (outflow SiO₂ 71-76 wt%) erupted from the Silver Creek Caldera, southern Black Mountains (SBM), AZ (Ferguson et al 2013); the ~18.9 Ma, trachytic CCT (63-68% SiO₂), the only large regional pre-PST ignimbrite (Buesch & Valentine 1986; Lidzbarski 2014), likely had a nearby source. Intermediate lavas, similar in composition to the CCT, dominate the pre-CCT section in the SBM; mafic lavas are present but sparsely exposed below the CCT. Fall deposits are exposed below CCT and between CCT and PST in the SBM and in the Transition Zone at Kingman, AZ. Buesch (1993) suggested the deposits at Kingman directly enveloping the CCT are part of the CCT eruptive unit—a correlation strongly supported by nearly identical geochemistry (Wood et al. (2016); Scheland et al. (2016)). The higher ash fall at Kingman, immediately below PST, was correlated with the Fort Rock Creek Rhyodacite tuffs (FT; fall and pyroclastic flow; Fuis, 1973, 1996) on the Colorado Plateau (Buesch, 1993). Our data for upper Kingman and FT tuffs support this correlation (e.g. SiO₂66-70 wt%). In contrast, the SBM fall deposits are more silicic than CCT or Kingman FT deposits (73-75 wt%).

Relatively low Zr, Zr/Sr, and Ti in biotite suggest the fall deposits in SBM, FT, and upper Kingman reflect magmas from cooler, wetter systems than those of CCT, PST, and pre-PST lavas of the Black Mountains (Miller et al., 2014; McDowell et al., 2014; Scheland et al., 2016). Our data may thus suggest three distinct petrogenetic systems in the NCREC and the adjacent, less extended, Transition Zone and Plateau: the SBM system (CCT, PST, lavas), which evolved from compositionally intermediate to silicic but remained relatively hot and dry; the FT (Plateau) system, cooler and wetter than SBM magmas; and the system with unknown source represented by the fall deposits in the SBM – highly silicic, cool and wet. We will present models of the contrasting sources and evolution of these systems using trace elements and Rhyolite-MELTS (Gualda et al. 2012).