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. 167 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 a metasomatized, moderately heterogenetic Proterozoic mantle source ( 87/86Sr of ~0.704-0.7055, εNd of -2 to 1) that mixed with and directly melted a dominantly mafic, low radiogenic Pb lower crustal basement with only minor felsic mid-upper crustal additions (<15% assimilation). Numerical simulations of thermal requirements for the SJVF necessitated 1.5 – 2x basalt for every volume of intermediate magma. A new energy constrained AFC model based on these Sr, Nd, and Pb analyses confirms the validity of the previous simulations. Taken together, this isotopic data and these new calculations suggest that the simplest explanation for the initiation of the SJVF cooling of North American lithosphere coupled with metasomatism during the Laramide 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.