PETROGRAPHY AND GEOCHEMISTRY OF INTRACALDERA IGNIMBRITE MATRIX AND TUFF BLOCKS: IMPLICATIONS FOR ERUPTION OF PEACH SPRING TUFF AND CALDERA COLLAPSE

Peach Spring Tuff (PST) is a supereuption-scale, 18.8 Ma ignimbrite that erupted from a caldera now exposed as tectonic fragments at Silver Creek, AZ, and near Eagle Peak (EP) in the Sacramento Mountains, CA (Ferguson et al 2013). Petrography and geochemistry of tuff blocks and matrix within intracaldera meso- to megabreccia at EP illuminate eruption progress and caldera collapse.

PST outflow near Kingman, AZ, comprises 5 zones (Ferguson & Cook 2015); geochemical and petrographic characterization reveals that the lower 4 zones are uniform in whole rock composition and phenocryst assemblage (Sa>>Pl, Bt±Hbl, mostly euhedral or fragmental)(Barry et al 2015). WR composition is uniformly ~74 wt% SiO₂, 0.30 TiO₂, 13.8 Al₂O₃, 1.5 Fe₂O_3t, 0.05 P₂O₅, 75 ppm Sr, 170 Ba, and 250 Zr, similar to reported pumice values (Pamukcu et al 2013). Uppermost zone 5 comprises phenocryst-rich trachyte (30-40%, strongly embayed) compositionally identical to intracaldera tuff at Silver Creek (65-70 wt% SiO₂, 15-17 Al₂O₃, 0.4-0.6 TiO₂, 2-4 Fe₂O_3t, 0.1-0.2 P₂O₅, 100-500 ppm Sr, 500-3000 Ba, 480-650 Zr; Pamukcu et al 2013, Foley et al 2014).

Both EP tuff blocks and matrix are rich in Sa phenocrysts with subordinate Pl, Bt, and Hbl, totaling ~20-40%. Fsp are commonly embayed. All samples are strongly affected by alkali exchange (K₂O commonly near 10 wt%, Na₂O ~1.5%); most other elements are fairly uniform and probably are similar to unaltered concentrations (mean matrix & block, respectively: 67.0, 67.7 wt% SiO₂; 0.38, 0.37 TiO₂; 15.0, 15.6 Al₂O₃; 3.1, 2.8 Fe₂O_{3 t}; 0.13; 0.13 P₂O₅; 240, 420 ppm Sr; 1060, 1220 Ba; 440, 510 Zr). Very similar compositions and textures suggest that blocks and matrix represent essentially the same magma, with blocks erupted slightly earlier and then incorporated into breccia during subsequent eruption of matrix material. Both are very similar to intracaldera trachyte tuff at Silver Creek and to PST outflow zone 5. We propose that crystal-rich trachyte was the magma that remained in the chamber after eruption of almost all of the outflow. Presence of late-stage trachyte blocks within intracaldera breccia indicates that caldera collapse was occurring during the final eruption stage; it may suggest that collapse was not triggered by the initiation of the supereruption, but rather was delayed until final stages of the process.