This is for completion of a petrologic and geochronologic investigation of caldera-related volcanism of the southeast San Juan volcanic field. The platoro-Summitville caldera comples, which includes voluminous pre- and postcaldera andesite to dacite volcanoes, records the early development of the San Juan field. The relatively mafic compositions of the Treasure Mountain ash- flow tuffs and their association with postcaldera andesite lavas, indicates that this magmatic system represents a primitive stage in the growth of the large silicic magma body which formed subsequently under the central and western San Juan caldera clusters. This is a field, petrologic, and isotopic study of the entire caldera-related volcanic sequence of the southeast San Juan field, including the Masonic Park Tuff. Although the basic geochemical characterization of much of the sequence is nearly complete, additional isotopic analyses (Pb, Sr and Nd) will be completed and further petrologic work will be done on the tuffs.
