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Publication Date

Spring 2015

Degree Type

Thesis - Campus Access Only

Degree Name

Master of Science (MS)

Department

Geology

Advisor

Jonathan Miller

Keywords

Hf, LASS, O, Supereruptions, Zircon

Subject Areas

Geology

Abstract

The 18.8 Ma Peach Spring Tuff is a 640 km3 ignimbrite that was erupted from the Silver Creek caldera in the Southern Black Mountains, Arizona. This is the only Miocene supereruption in the Colorado River extensional corridor and is part of the Southern Black Mountains volcanic center (SBMVC) that also includes pre- and post-caldera extrusives and post-caldera intrusives. Laser ablation split stream (LASS) Hf isotope and U-Pb age analyses on zircon and coupled zircon O isotope analyses were carried out on the Peach Spring Tuff and SBMVC rocks to better understand how the ignimbrite was assembled and generated, what sources fed the magma body, and the relationship between the Peach Spring Tuff and the associated pre- and post-caldera units. The isotope data show that the units of the SBMVC are heterogeneous mixtures of Proterozoic crust and 50-90% enriched mantle and become more juvenile over time (increasing εHf, decreasing δ18O). The Peach Spring Tuff has the most crust-like values of the system (lowest εHf and highest δ18O) and is the most isotopically homogenous unit in the SBMVC, despite its large volume. This is consistent with an origin by fractionation from a less silicic parent. LASS analysis shows that Proterozoic zircons are rare, and where present, do not have resolvable Miocene overgrowths. This indicates that these Proterozoic zircons were incorporated late in the melt and that SBMVC magmas were initially above zircon saturation.

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