Abstract The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km 3 ) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.

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Abstract The widespread igneous rocks in the western United States have now been studied for more than a century; they provide primary geologic constraints on many types of ore deposits, key information on regional geologic history, and the basis for development of broad concepts applicable to global magmatism. Studies have been especially intense in the past few decades, fueled by societal needs such as volcanic hazard evaluations and the search for geothe Rmal and mineral resources. Also important have been an explosion of new techniques in geochronology and isotope geochemistry, increased availability and precision of mineral and rock chemical analyses, and detailed field structural studies of batholiths and stratigraphic studies of volcanic fields. Improved understanding of eruptive mechanisms has resulted from combined studies of historic volcanism and prehistoric deposits, and the rich interactions between studies of igneous activity and innovative plate tectonic concepts have led to new ideas on the origins of continental-margin magmatism.