From ignimbrite to batholith, northeastern San Juan Mountains, Colorado; Bonanza, Cochetopa Park, and North Pass calderas

The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are approximately 26 regional ignimbrites (each 150-5000 km (super 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.