Paper No. 3
Presentation Time: 1:45 PM


JACOB, Kristin H., Geological Sciences, University of Colorado, Campus Box 399, Boulder, CO 80309, FARMER, G. Lang, Department of Geological Sciences, University of Colorado, Boulder, CO 80309 and COSCA, Michael A., USGS, Box 25046, Denver, CO 80225-0046,

Whole-rock major- and trace-element geochemistry, Sr and Nd isotopic data, and 40Ar/39Ar age determinations were used to assess the petrogenesis of an intermediate to high-silica rhyolite suite of extrusive rocks at the ~28 Ma Never Summer (aka Braddock Peak) igneous complex (NSIC), north-central Colorado. These data were used to determine the nature of the relationship between the suite of extrusive rocks and the adjacent plutonic rocks of the Never Summer batholith. The latter is comprised of the intermediate, chemically zoned Mt Richthofen stock (MRS; εNd -0.5 to -5.8) and the younger, highly silicic Mt Cumulus stock (MCS; SiO2 77 wt %, εNd ~-5.5). The studied volcanic rocks are from a continuous sequence found in the “Crater” area of Rocky Mountain National Park. The oldest preserved volcanic rocks are a set of andesite lava flows (29.14 to 28.76 Ma;εNd ~ -1.8 to -4.7) deposited directly on Precambrian basement rocks. These are overlain by lahars and rhyolite flows. The volcanic activity culminated in the eruption of ~ 12 km3 of high-silica topaz-bearing rhyolites (~28.05 Ma; εNd ~-5.0). The Sr isotopic compositions of both the MCS and rhyolites scatter around the same 28 Ma reference isochron and have “gull-shaped” REE patterns. We suggest that both the MCS and rhyolites were derived from highly silicic parental melts that rose to shallow levels in the upper crust in this region. These parental melts were most likely the products of partial melting of garnet-absent mafic lower crust, based on a direct assessment of the mineralogy, chemical, and isotopic compositions of the lower crust beneath north-central Colorado afforded by lower crustal xenoliths entrained in nearby Devonian kimberlites. The andesites found at the Crater contain resorbed qtz-feld glomerocrysts that may record interaction between early formed granite intrusions in the upper levels of the magma system and later underplated mafic, mantle-derived magmas. If so, then it is possible that the spectrum of intermediate composition rocks found at the NSIC were produced in situ in the upper crust. Overall, a connection between the volcanic and plutonic realms exists in that both represent the result of the injection of mafic and felsic magmas into the upper crust, but no direct genetic relationship need exist between any of the igneous rocks preserved at the NSIC.