An Investigation of Volcanism, Pluton Emplacment, and Zircon Systematics by U-Pb Geochronology (2010)

Undergraduate: Kyle Samperton

Faculty Advisor: Drew Coleman
Department: Geology

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A major goal in the field of petrology is to improve our understanding of the fundamental relationships between volcanic and plutonic rocks. In this study, high-precision U-Pb zircon geochronology is integrated with 40Ar/39Ar thermochronometry to complete the following objectives: 1) elucidate the temporal evolution of the Latir volcanic locus in the Southern Rocky Mountain volcanic field (SRMVF) by obtaining a U-Pb age on the Amalia Tuff, a ~500 km^3 ignimbrite that represents the only Tertiary ash flow tuff in the region; 2) employ geochemical data and geo-/thermochronology to identify a potential intrusive equivalent to the Amalia Tuff and describe their time-space relationship; and 3) explain systematic normal discordance in U-Pb ages by intermediate daughter product disequilibrium, and draw evidence from the literature to support such a claim. A 206Pb/238U age of 25.43 +/- 0.06 Ma was determined for the Amalia Tuff, statistically indistinguishable from the 40Ar/39Ar dates on both the Amalia Tuff and the Virgin Canyon pluton (25.39 +/- 0.04 Ma and 25.38 +/- 0.04 Ma, respectively). These preliminary results support short-lived magmatic residence in the upper crust prior to eruption and subsequent pluton crystallization. Systematic normal discordance observed in volcanic-derived zircons is sourced to protactinium-231 disequilibrium relative to uranium-235, resulting in apparently older 207Pb/235U ages relative to 206Pb/238U values.