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.
Preferred Presentation Type: Oral