THE AURORA VOLCANIC FIELD, CALIFORNIA-NEVADA - OXYGEN FUGACITY CONSTRAINTS ON THE DEVELOPMENT OF ANDESITIC MAGMA

The Aurora volcanic field, located along the northeastern margin of Mo no Lake in the Western Great Basin, has erupted a diverse suite of hig h-K and shoshonitic lava types, with 48 to 76 wt% SiO2, over the last 3.6 million years. There is no correlation between the age and composi tion of the lavas. Three-quarters of the volcanic field consists of ev olved (<4 wt% MgO) basaltic andesite and andesite lava cones and flows , the majority of which contain sparse, euhedral phenocrysts that are normally zoned; there is no evidence of mixed, hybrid magmas. The aver age eruption rate over this time period was similar to 200 m(3)/km(2)/ year, which is typical of continental arcs and an order of magnitude l ower than that for the slow-spreading mid-Atlantic ridge. All of the A urora lavas display a trace-element signature common to subduction-rel ated magmas, as exemplified by Ba/Nb ratios between 52 and 151. Pre-er uptive water contents ranged from 1.5 wt% in plagioclase-rich two-pyro xene andesites to similar to 6 wt% in a single hornblende lamprophyre and several biotite-hornblende andesites. Calculated oxygen fugacities fall within -0.4 and +2.4 log units of the Ni-NiO buffer. The Aurora potassic suite follows a classic, calc-alkaline trend in a plot of FeO T/MgO vs SiO2 and displays linear decreasing trends in FeOT and TiO2 w ith SiO2 content, suggesting a prominent role for Fe-Ti oxides during differentiation. However, development of the calc-alkaline trend throu gh fractional crystallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the Ni-NiO buffer. Nor can fractionation of hornblende be invoked, since it has t he same effect as titanomagnetite in depleting the residual liquid in ferric iron, together with a thermal stability limit that is lower tha n the eruption temperatures of several andesites (similar to 1040-1080 degrees C; derived from two-pyroxene thermometry). Unless some progre ssive oxidation process occurs, fractionation of titanomagnetite or ho rnblende cannot explain a calc-alkaline trend in which all erupted lav as have oxygen fugacites greater than or equal to the Ni-NiO buffer. I n contrast to fractional crystallization, closed-system equilibrium cr ystallization will produce residual liquids with an oxygen fugacity th at is similar to that of the initial melt. However, the eruption of ne arly aphryic lavas argues against tapping from a magma chamber during equilibrium crystallization, a process that requires crystals to remai n in contact with the liquid. A preferred model involves the accumulat ion of basaltic magmas at the mantle-crust interface, which solidify a nd are later remelted during repeated intrusion of basalt. As an end-m ember case, closed-system equilibrium crystallization of a basalt, fol lowed by equilibrium partial melting of the gabbro will produce a calc -alkaline evolved liquid (namely, high SiO2 and low FeOT/MgO) with a r elative f(o2) (corrected for the effect of changing temperature) that is similar to that of the initial basalt. Differentiation of the Auror a magmas by repeated partial melting of previous underplates in the lo wer crust rather than by crystal fractionation in large, stable magma chambers is consistent with the low eruption rate at the Aurora volcan ic field.