THE EVOLUTION OF THE PEACH SPRING TUFF AND SILVER CREEK VOLCANIC CENTER AS REVEALED BY HF AND O ISOTOPES IN ZIRCON

The 18.8 Ma Peach Spring Tuff (PST) is the only supereruption-sized ignmibrite (700 km³DRE) in the Colorado River Extensional Corridor (CREC), an area of large-magnitude Miocene extension and magmatism. The PST was erupted from the Silver Creek Caldera in the southwestern Black Mountains, where pre- and post-caldera volcanic rocks and caldera-related resurgent intrusions are well-exposed; these rocks thus provide a detailed record of magma evolution leading up to and following the paroxysmal caldera-forming supereruption.

The ignimbrite is zoned from high-SiO₂ rhyolite (dominant in outflow) to crystal-rich trachyte (proximal outflow and intracaldera) and is characterized by high K, Zr, and REEs. Whole pumice and fiamme from all compositions have a very narrow range of isotopic variability (ε_Nd -11.5 to -11.7, ε_Hf -13.8 to -14.3, ⁸⁷Sr/⁸⁶Sr_i 0.7108-0.7121). High-precision CA-TIMS as well as ion probe U-Pb zircon dating has documented little inherited or antecrystic zircon. New zircon Hf and O from high-SiO₂ rhyolite outflow pumice as well as crystal-rich trachyte pumice also show limited and identical isotopic variability with approximately 50% local crustal component (ε_Hf = -13.8±1.5, N=62 excl. 1 Proterozoic zircon; δ18O 6.4±0.4‰, N=68). Resurgent intrusions as well as pre- and post-caldera volcanic rocks have distinct whole rock and zircon ε_Hf values with somewhat wider variation (generally with > mantle component).

The data reinforce and refine earlier work (McDowell et al. 2012; GSA Abstracts v. 44/7, No. 128-4; Frazier et al. 2012; GSA Abstracts v. 44/7, No. 128-5) and indicate that: (1) the PST magma chamber was isotopically homogeneous down to the scale of zircon despite the wide range of bulk compositions, which requires vigorous homogenization and/or limited isotopic variability in the melt source; (2) anatexis occurred at relatively high temperatures (well above zircon saturation); (3) fractionation to high-silica rhyolite occurred in the shallow PST magma chamber; (4) eruption drained the magma chamber of PST magma efficiently, and post-caldera intrusions represent a distinct pulse of magmatism and not unerupted crystal mush.