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.

Early Proterozoic supracrustal rocks near Gunnison and Salida, Colorado, include sequences of tholeiitic metabasalt, metarhyolite to metadacite, and interbedded volcaniclastic turbidite. These rocks were intruded by synchronous gabbroic sheets, complexly folded and metamorphosed in upper greenschist to upper amphibolite facies, and intruded by plutons ranging from quartz diorite to granite. U-Pb ages of zircons show that an early period of volcanism in the Gunnison area occurred from 1,770 to 1,760 Ma and was followed by emplacement of plutons from 1,755 to 1,750 Ma. A younger sequence of volcanic rocks was formed in both the Gunnison and Salida areas between 1,740 and 1,730 Ma. In the Gunnison area these rocks were intruded by major plutons from 1,725 to 1,714 Ma. Near Howard, Colorado, southeast of Salida, metarhyolite yielding ages of 1,713 and 1,668 Ma is believed to be part of the younger sequence. Late, post-tectonic granite plutons were emplaced in both areas from 1,700 to 1,670 Ma. The age data, petrography, and geochemistry of these rocks indicates that they are part of a broad belt of juvenile, arc-related terranes, exposed from southern California across Arizona, New Mexico, Colorado, and known in the subsurface as far east as western Missouri, that was accreted to the southern edge of the continent during the Early Proterozoic.