DECONSTRUCTING THE EVOLUTIONARY HISTORY OF SUMMER COON VOLCANO, COLORADO: EVIDENCE FROM GEOCHEMISTRY AND RADIOGENIC ISOTOPES

On the western margin of the Northern Rio Grande Rift lies the Summer Coon volcano, a heavily-dissected stratovolcano active from 35 to 32 Ma (Lipman, 1968). Significant erosion has removed most of its edifice, leaving behind a central intrusive complex and hundreds of radial dikes at the surface. Compositions of the intrusive and extrusive rocks range from basaltic andesites to rhyolites. Previous work on the Summer Coon divided the volcanic stratigraphy into three units (Early Mafic, Middle Silicic, Late Intermediate; Mertzman, 1971), each containing a sequence of volcanic deposits correlated by composition to intrusive rocks of the central complex. Simple fractional crystallization alone cannot explain this compositional sequence; rather, it is likely the result of both open system and magma chamber differentiation processes.

To investigate the mafic-silicic-intermediate sequence in detail, we obtained high-precision isotopic data (i.e., Sr-Nd-Hf-Pb compositions) from each of the three units. Preliminary isotopic results via MC-ICP-MS for ⁸⁷Sr/⁸⁶Sr (0.705119 to 0.705862), ϵNd (-8.44 to -8.98), and ϵHf (-6.13 to -7.15) are relatively similar. Pb isotope ratios for the Early Mafic and Middle Silicic units are nearly identical (²⁰⁶Pb/²⁰⁴Pb = 17.561 and 17.584, ²⁰⁷Pb/²⁰⁴Pb = 15.482 and 15.485, and ²⁰⁸Pb/²⁰⁴Pb = 37.118 and 37.124, respectively), whereas the Late Intermediate unit differs slightly in its Pb isotopic composition (²⁰⁶Pb/²⁰⁴Pb = 17.307, ²⁰⁷Pb/²⁰⁴Pb = 15.456, and ²⁰⁸Pb/²⁰⁴Pb = 36.865). Inverse modeling of geochemical and isotopic data was utlized to investigate various scenarios for producing the mafic-silicic-intermediate sequence. Based on the observed major and trace element trends and preliminary modeling results, fractional crystallization was the dominant differentiation process and there has been little crustal interaction. All units are possibly related to a parental basalt by crystal fractionation, although back-mixing of the Early Mafic and Middle Silicic magmas may have produced the Late Intermediate compositions. Future work includes additional sample analyses to further evaluate isotopic trends and develop a comprehensive, self-consistent model of the evolution of Summer Coon volcano.