Intermediate lavas from the middle Miocene McDermitt caldera complex, Nevada-Oregon, have very high FeO* (= total Fe as FeO; 9.1 to 10.2 wt %) and low MgO (0.4 to 2.1 wt %) contents at SiO2 contents of 59.5 to 62 wt %. With the exception of significantly higher K2O contents (3.1 to 4.7 wt %), these lavas are compositionally very similar to icelandite (“tholeiitic andesite”) from oceanic areas. In addition to the voluminous comendite tuffs of the complex, a thin unit of densely welded crystal-rich pantellerite tuff of probable air-fall origin is exposed on the northern wall of the McDermitt caldera. The pantellerite tuff contains 1.5% aenigmatite phenocrysts. The high FeO*/MgO ratios of the icelandites and the peralkaline nature of the silicic rocks suggest differentiation under conditions of low fO2 and fH2O. The existence at McDermitt of a continuous series of rocks ranging from potassic icelandite to comendite and pantellerite is consistent—as are other geologic, geochemical, and geophysical data—with the derivation of the icelandites and ultimately of the voluminous peralkaline silicic rocks of the complex from mafic magma generated from upwelling mantle material. Labile U released during the crystallization of late subvolcanic bodies of peralkaline silicic magma may have provided an appreciable proportion of the epigenetic U now present in the upper part of the caldera complex. Reducing conditions produced by the very Fe-rich glassy icelandites may have contributed to the fixation of U at the Aurora and Bretz prospects.

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