Geological Society of America

T153 Illuminating Felsic Origins: Using Novel Multiple-Method Approaches to Investigate the Birth of Silicic Magmas (Posters)

Monday, 28 October 2013
9:00 AM - 6:30 PM
Hall D

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

Paper number: T153-3
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Sarah Overton Presenting Author
San Jose State University, San Jose, CA
William O. Frazier
Vanderbilt University, Nashville, TN
Susanne M. McDowell
Vanderbilt University, Nashville, TN
Marsha Lidzbarski
San Jose State University, San Jose, CA
Jonathan S. Miller
San Jose State University, San Jose, CA
Calvin F. Miller
Vanderbilt University, Nashville, TN
Christopher M. Fisher
Washington State University, Pullman, WA
Rita C. Economos
University of California - Los Angeles, Los Angeles, CA
Gareth R. Davies
VU University Amsterdam, Amsterdam

Abstract

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.