PB, SR, AND ND ISOTOPIC INSIGHTS INTO THE ORIGINS OF THE SAN JUAN VOLCANIC FIELD

The cause of the Oligocene San Juan Volcanic Field (SJVF), southwestern Colorado is debated. The results of a three-year systematic campaign to locate and characterize potential parental mafic source magmas and lower crustal contaminants of this ignimbrite province are presented. 154 samples were collected from three previously unsampled reservoirs: 1-basaltic, basaltic-andesitic, and alkalic dikes from San Juan volcanic and intrusive centers on the periphery of the extensive ignimbrite cover, 2- xenoliths included in these deposits, and 3-Proterozoic basement exposed within the Needle Mountains uplift. A subset of representative, fresh samples from these reservoirs were picked for geochemical (n = 50) and isotopic (n = 32) analysis. Mixing trends indicate that the SJVF can be explained as melts produced from a metasomatized, moderately heterogenetic Proterozoic mantle source (⁸⁷Sr/⁸⁶Sr of ~0.704-0.7055, εNd of -2 to 1) that heated and melted a dominantly mafic, low radiogenic Pb lower crustal basement with only minor (< 10%) felsic mid-upper crustal assimilation. A new energy constrained assimilation-fractional crystallization (EC-AFC) model based on these Sr, Nd, and Pb analyses suggests basalt volumes of 1.5-2x evolved magma volumes. Taken together, this isotopic data and these new calculations suggest that the simplest explanation for the formation of the SJVF is that cooling and metasomatism of North American during the Laramide “pre-fluxed” the lithospheric mantle, which coupled with enrichment from earlier orogenies, primed southwestern Colorado for extensive melting once the Farallon slab retreated. The volumes of observed intermediate magma and calculated parental basalt and their isotopic signatures can be explained without requiring a concurrent extensional tectonic regime, lithospheric delamination, or substantial asthenospheric melting.