Petrogenesis of the Superstition-Superior Volcanic Area as Inferred from Strontium- and Oxygen-Isotope Studies

摘要

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.