GEOCHRONOLOGY AND TRACE ELEMENT ANALYSIS OF PEACH SPRING TUFF ZIRCONS AND THEIR BEARING ON GROWTH OF THE PEACH SPRING TUFF MAGMA CHAMBER AND ERUPTION

The Peach Spring Tuff (PST) is a voluminous (>600 km³), zoned ignimbrite (trachyte to high-SiO₂ rhyolite) that is exposed widely in eastern California, western Arizona, and southernmost Nevada, and which was erupted from a caldera in the southwestern Black Mountains, AZ (Silver Creek caldera). We are undertaking combined U-Pb SHRIMP and CA-TIMS analysis as well as trace element analysis on zircon from individual pumice clasts from the PST to evaluate the growth history of zircon in the magma system.

U-Pb SHRIMP ages of zircons from 4 individual pumice clasts from two outflow localities of PST yield variable weighted means, ranging from ca. 18.8 Ma to 18.5 Ma with 2 samples having zircons that scatter to several hundred thousand years older or younger than these means (up to 1 Ma total spread). A modest number of the older grains are plausibly recycled antecrysts, but we suspect that the youngest zircons may have experienced lead loss; CA-TIMS work is under way on a subset of the same zircons to better evaluate the cause of the dispersion. Sparse Proterozoic zircons (ca. 1.7 Ga) were also found and lack Miocene zircon overgrowths (even on outer crystal surfaces of unsectioned zircons).

Trace element analysis and CL-imaging of zircons from pumices from the base and very top of the ignimbrite from exposures near Yucca, AZ show interesting contrasts. The zircons from the upper pumice have bright rims on darker cores, have low U (<20 ppm) and Hf (<8000 ppm), and high Ti-in-zircon model temperatures. Zircons from the basal pumice in this location lack these features, and elemental characteristics are otherwise similar to zircons from other outflow pumices. The upper sample represents the last erupted material and presumably correlates with the deepest tapped portions of magma chamber. We interpret the bright rims to indicate thermal rejuvenation at the base of the magma chamber just prior to eruption. Similar bright rims have been seen in zircons from the intracaldera tuff but not in zircons from individual pumice clasts in the outflow sheet. The lack of Miocene overgrowths on Proterozoic zircons in pumice indicates that these are not grains inherited from the source but xenocrysts incorporated into the magma chamber shortly before eruption, perhaps during foundering and disaggregation of the Proterozoic roof and walls of the caldera.