Characterizing Geographic Variability in Magma Composition and Eruptive Style During Voluminous Mid-Tertiary Magmatism in the Northern Great Basin Using Zircon U-Pb Geochronology and Phenocryst O Isotopic Data

Following a period of magmatic quiescence in the early Tertiary, magmatism returned to the western U.S. with vigor, migrating southward across the Cordillera from the Eocene to the early Miocene. Although often referred to as the "ignimbrite flareup," the large caldera-forming silicic eruptions associated with Great Basin phase of this event are mostly restricted to the late Oligocene to early Miocene of central Nevada, while voluminous Eocene magmatism in northeastern Nevada manifested instead as dikes, plutons, and flow-dome complexes with few small-volume tuffs. The 34 Ma Caetano caldera, one of the northernmost calderas of central Nevada, is characterized by magmatic δ¹⁸O values of ~10-10.5‰ (based on analysis of zircon, quartz, and plagioclase) and numerous inherited (mostly Proterozoic) zircons, consistent with a substantial metasedimentary contribution to the magma. In this respect it is similar to non-caldera forming magmas further north and east, including those from Eocene mid-crustal depths now exposed in the Ruby Mountains core complex. In contrast, the 25 Ma Elevenmile-Poco Canyon caldera complex in the Stillwater Range, one of the westernmost and youngest caldera systems, is characterized by magmatic δ¹⁸O values of 5.5-7.5‰ and a lack of inherited zircon, consistent with assimilation of little sedimentary material. Intervening caldera systems show a progressive southwestward decrease in magmatic δ 18O values, consistent with a decreasing metasedimentary contribution to their source magmas. These patterns may reflect profound changes in crustal architecture across the mid-Tertiary Great Basin. Eocene northeastern Nevada lay in the hinterland of the Cretaceous Sevier fold-thrust belt, where Paleozoic sedimentary rocks were buried to depths >25 km in a crust 40-50 km thick. This thick metasedimentary crustal column may have conspired to stall silicic magmas in the crust and prevent the formation of large eruptible magma chambers. Formation of increasingly numerous calderas to the south and west may have been promoted by a more vigorous magmatic pulse ca. 25 Ma, and permitted by a thinner, overall more mafic crust that was less effective at trapping silicic melts at mid-crustal depths. Additional U-Pb geochronology and radiogenic isotope measurements are currently being collected to evaluate this hypothesis and better characterize geographic variations in the character of mid-Tertiary Great Basin magmatism.