Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States

Abstract

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830-1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low f H 2 O and low f O 2 ) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe-Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.

First evidence for Permian-Triassic boundary volcanism in the Northern Gemericum: geochemistry and U-Pb zircon geochronology

“…The metarhydacites of Group II have significantly lower concentrations of REE, Y, Nb and Zr, and as a result they plot in the volcanic arc field. Similar contrasting types of silicic volcanic rocks were described by Christiansen & McCurry (2008) from Cenozoic volcanism of the western Cordillera. According to their interpretation, they came from different "mantle parents", i) mantle wedge above subduction zone (linkage to subduction heritage); ii) partial melting in/or above a mantle plume (continental rift and "hotspots").…”

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

“…Several authors have suggested that the magmas parental to both topaz rhyolites and syenogranites related to molybdenum mineralization in the Rocky Mountain region were related to episodes of lithospheric extension (Stein and Hannah 1985;Ludington and Plumlee 2009). The fact that the Never Summer igneous complex was related to bimodal magmatism is consistent with this interpretation (Christiansen and McCurry 2008) despite available field evidence that Cenozoic extension in this region was small in magnitude and occurred post-28 Ma (Chapin and Cather 1994). Even if extension and crustal exhumation accompanied magmatism at the Never Summer igneous complex, exhumation alone cannot trigger dehydration melting of amphibolite in the lower crust without the addition of heat, presumably through the incursion of mantle-derived magmas (Thompson and Connolly 1995).…”

Evidence for Cyclical Fractional Crystallization, Recharge, and Assimilation in Basalts of the Kimama Drill Core, Central Snake River Plain, Idaho: 5.5-Million-Years of Petrogenesis in a Mid-crustal Sill Complex

“…The region contains a record of continuous bimodal volcanism extending over 17 Ma Coble and Mahood, 2012;Henry et al, 2017) to the present, and documents the migration of timetransgressive rhyolitic volcanism from the Bruneau-Jarbidge volcanic center (circa 12 Ma) to its present location beneath the Yellowstone Plateau (Pierce and Morgan, 1992;Anders et al, 2009). Interaction between the mantle hotspot and overlying continental lithosphere has resulted in large rhyolite calderaforming eruptions, followed by eruption of smaller basaltic shield volcanoes (Bonnichsen et al, 2008;Christiansen and McCurry, 2008;McCurry and Rodgers, 2009). Post-caldera basaltic flows form a veneer over the rhyolite ash flows and lavas and mask the complete volcanic record.…”

“…Previous workers have demonstrated that chemical variation between SRP basalt flows is a result of fractional crystallization, crustal contamination, and partial melting occurring within the mid-crustal sill at pressures of ∼0.6 GPa and temperatures of 1,205 ± 27 • C (Kuntz et al, 1992;Shervais et al, 2006;Christiansen and McCurry, 2008;McCurry and Rodgers, 2009;Miller and Hughes, 2009;Putirka et al, 2009). For Kimama drill core basalts specifically, Bradshaw (2012) estimated pre-eruptive crystallization occurred at 0.2-0.5 GPa (7-17 km) and between 1,155 and 1,255 • C. Putirka et al (2009) argue for a three-stage process to explain the entire range of SRP lava compositions.…”