Granulite Facies and Related Xenoliths from Colorado-Wyoming Kimberlite

Deep crustal anatexis, magma mixing, and the generation of epizonal plutons in the Southern Rocky Mountains, Colorado

“…Rare earth element, Sr, and Y mineral-melt partition coefficients used were those estimated by Bédard (2006) and Nehring et al (2009) for the anatexis of mafic granulites (see Supplementary Material). The average source rock trace element abundances used were those from the "high La/Yb N " State Line diatreme xenoliths (Farmer et al 2005), and the range of mineral modes for these xenoliths were from Bradley (1985). As expected, partial melts of garnetbearing lithologies produce much higher La/Yb, lower absolute HREE abundances, and higher Sr/Y than melts from garnet-absent mafic crust (Fig.…”

“…These xenoliths not only sample Precambrian crust in the vicinity of the Never Summer Mountains, but also provide a snapshot of the crustal composition prior to the onset of Late Cretaceous and younger magmatism in this region. The State Line xenoliths include two-pyroxene (± amphibole) granulites, two-pyroxene-garnet granulites, and cpx-garnet granulites (Bradley 1985) which demonstrate that the Devonian lower crust in this region was mafic in composition and dominated by garnet-bearing lithologies in its lowermost portions. Thermobarometry from the garnet-bearing xenoliths yields last equilibration pressures and temperatures of ~1.1 GPa and ~800 °C, which requires a Devonian crustal thickness of at least 35-40 km (Bradley 1985;Farmer et al 2005).…”

“…The State Line xenoliths include two-pyroxene (± amphibole) granulites, two-pyroxene-garnet granulites, and cpx-garnet granulites (Bradley 1985) which demonstrate that the Devonian lower crust in this region was mafic in composition and dominated by garnet-bearing lithologies in its lowermost portions. Thermobarometry from the garnet-bearing xenoliths yields last equilibration pressures and temperatures of ~1.1 GPa and ~800 °C, which requires a Devonian crustal thickness of at least 35-40 km (Bradley 1985;Farmer et al 2005). U-Pb ages from zircon present in mafic composition xenoliths range from ~1.6 to 1.73 Ga and overlap the ages of intermediate to felsic composition Paleoproterozoic crust exposed in this region (Farmer et al 2005).…”

“…a Melting of a garnet-bearing two-pyroxene source with approximate modal compositions from Farmer (2005); 20.9 % plagioclase, 31 % orthopyroxene, 24 % clinopyroxene, 10.9 % garnet, 6 % amphibole, 0.1 % apatite. b Melting of a garnet-free two-pyroxene source with approximate modal compositions from Bradley (1985); 50 % plagioclase, 20 % orthopyroxene, 24 % clinopyroxene, 4 % amphibole, 2 % apatite product of alkali feldspar fractionation during shallow differentiation. This conclusion is consistent with the fact that even 10 % partial melting (not shown) of typical State Line two-pyroxene granulite xenoliths only generates melts with modest negative Eu anomalies (Eu/Eu* ≥ 0.9) and leaves a residual solid with Eu/Eu* of only 1.05-1.08, using mineral-melt partition coefficients from Nehring et al (2009).…”

Neoproterozoic kimberlite emplacement in the Front Range, Colorado

“…Petrologic and geochemical studies of these xenoliths have been used to constrain the composition and age of subcontinental lithosphere in northern Colorado and southern T47yoming (Eggler and McCallum, 1974;Eggler et al, 1979;Bradley, 1984;Lester and Farmer, 1998). The presence in several kimberlite pipes of sedimentary xenoliths containing Cambrian, Ordovician, and Silurian marine fossils, coupled with the absence of younger sedimentary xenoliths, was used to suggest intrusion during the Late Silurian to Early Devonian (Chronic et al, 1969;McCallum et al, 1975;McCallum, 1991).…”