The San Juan volcanic field is made up of about 25,000 cubic km. of magma, making it one of the largest volcanic centers in the western U. S. Exceptional exposures exist of pre-, syn-, and post-caldera lavas and ash-flow tuffs. Little modern isotopic work has been done, and no Nd data exist for the field. Workers form six other institutions are currently involved in a variety of geologic studies. Recently controversy has arisen as to the depth of origin of some ash-flow tuffs centered on the San Juan field, and isotopic data should help provide a resolution. A comprehensive H, O, Sr, Nd, and Pb isotopic and petrologic study is planned, involving both lavas and ash-flow tuffs of the central caldera complex. The isotopic work will be aimed at understanding processes of magma formation and evolution, and will be firmly tied to field and petrologic data from this study and the work of others.
