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Earth Science Lecture Series

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ABSTRACT

The Pliocene-Pleistocene Aurora Volcanic Field (AVF) covers 325 km2, stretches across the California-Nevada border north of Mono Lake, and overlaps the Miocene Bodie Hills Volcanic Field (BHVF). It lies within the northwest-striking Walker Lane shear zone at the western margin of the Basin and Range province. 

Although the AVF lavas erupted well after the termination of arc-related volcanic activity in the area, their normalized incompatible element patterns show a continental arc-like signature. The AVF is comparable to to other Pliocene-Pleistocene volcanic fields within the Walker Lane, which also erupted in a post-subduction setting; however, the AVF tends to be more evolved compositionally (i.e., almost all rocks contain >50% silica), masking its initial mantle source signature and subsequent differentiation processes. Petrographic features suggest syn-magmatic open system processes, further obscuring the original geochemistry of the AVF rocks.

Isotope analysis was used to distinguish between an initial mantle magma contaminated during passage through the crust, versus one that had evolved by mineral crystallization as the magma cooled and solidified. While the AVF rocks’ trace element compositions show few signs of open system processes, the wide range of isotopic ratios indicate crustal assimilation. The BHVF and the AVF rocks have similar Pb isotope ratios; however, the AVF rocks have a higher 87Sr/86Sr ratio, a lower 143Nd/144Nd ratio, and stronger correlations between 87Sr/86Sr, silica, and δ18O, indicating that crustal assimilation was a more influential process in the AVF than in the BHVF.