Petrogenesis of the Superstition-Superior volcanic area as inferred from strontium- and oxygen-isotope studies

Apparent initial Sr⁸⁷/Sr⁸⁶ ratios of five ash-flow tuffs (0.7063 to 0.7139) and several mafic to silicic lavas (0.7055 to 0.7131) indicate that the magmas were derived below the base of the Precambrian granitic crust (0.7231 to 1.0906). Liquidus compositions in the system Q-Or-Ab-H₂O and oxygen-isotope geother-mometry suggest that the silicic magmas started to crystallize quartz, magnetite, and two feldspars in a water-undersaturated environment of high pressure (∼10 kb) and moderate temperature of at least 830°C.

During or after ascent into the crust, the magmas underwent varying degrees of crystal-melt re-equilibration. Measured plagioclase-biotite O¹⁸ fractionations (0.5 to 0.7) imply a temperature that is too high for the observed mineral assemblage, and the inference is that the two minerals did not crystallize in equilibrium. Prior to eruption, the upper part of the magma column assimilated crustal Sr such that the base of each ash flow is now enriched in Sr⁸⁷. In some cases, this assimilation was too rapid to allow crystal-melt equilibration of Sr isotopes. δO¹⁸ values for the magmas are within the range typical of similar magma types, indicating that no significant interaction took place between the melts and meteoric water; however, some rocks have exchanged oxygen isotopes with meteoric water at low temperatures after eruption.

Some of the magma appears to have had a long residence in the crust at lower pressure (1 kb) and temperature (750°C), because two of the ash-flow tuffs and one lava are greatly enriched in Sr⁸⁷ and have largely re-equilibrated under the P-T conditions of a shallow magma chamber. Even these, however, have retained evidence for a multistage genesis.