Eight large volume (100-300 km3) ash-flow tuffs wee erupted from a series of nested calderas in the central San Juan Volcanic Field during a period of less than 2 million years during the late Oligocene (26-28M. years). These ash flows and the intercalated volcanic and shallow plutonic rocks represent a series of stages in the development of a large silicic magmatic system, perhaps a batholith, in the crust of this region. Two ash-flow tuffs have been studied in detail and work on the other units is in progress. It has been possible to estimate temperature, pressure, and depth, as well as water, oxygen, sulfur, and halogen fugacities in the developing magmas using analyses of the phenocryst minerals. Numerical modeling has also been carried out. There appear to be three major magma types found in this region. The ash-flow tuffs were produced as a result of giant eruptions of a magnitude not observed in historic times. A careful study of these large volume volcanic products will enable comparison with plutonic systems of comperable size. The andesitic magmas apparently are closely connected both geochemically and thermally with the development of the silicic ash flows This work will continue the study of these rock units, and should contribute to the knowledge of explosive volcanic phenomena, geothermal energy, certain ore deposits and, more generally, processes operating in the continental crust.
