ERUPTIVE AND NON-ERUPTIVE CALDERAS, NORTHEASTERN SAN JUAN MOUNTAINS, COLORADO: WHERE DID THE IGNIMBRITES COME FROM?

The northeastern San Juan Mountains, the least studied portion of this well-known volcanic region, are the site of several newly identified and/or reinterpreted ignimbrite-caldera systems. The previously unrecognized North Pass caldera, in the Cochetopa Hills, was source of the 32.25-Ma Saguache Creek Tuff (~250 km10³). This regionally distinctive crystal-poor alkalic rhyolite helps fill a apparent previous gap in the southwestward migration from older explosive activity, from calderas along the N-S Sawatch locus in central Colorado (youngest, Bonanza Tuff at 32.9 Ma), to the culmination of Tertiary volcanism in the San Juan region, where large-volume ignimbrite eruptions started at ~29 Ma and peaked with the enormous Fish Canyon Tuff (5000 km3) at 28.0 Ma. The entire North Pass cycle, including small precursor tuffs, caldera-forming Saguache Creek Tuff, thick caldera-filling lavas, and a small-volume late tuff sheet, is tightly bracketed at 32.5-32.2 Ma. No large tuff sheets were erupted in the interval 32-29 Ma, but a previously unmapped cluster of dacite - rhyolite lava flows and small tuffs, associated with a newly recognized intermediate-composition intrusion 5x10 km across (largest subvolcanic intrusion in San Juan region), centered 15 km north of the North Pass caldera, mark a near-caldera-size silicic system active at 29.8 Ma. Crystal-rich flows in the cluster have mineral and chemical affinities to the Fish Canyon Tuff, recording early development of this regionally distinctive composition. In contrast to the completely filled North Pass caldera that has little topographic expression, no voluminous tuffs vented from the adjacent Cochetopa Park caldera that is morphologically beautifully preserved. Instead, Cochetopa Park subsided passively as the >500 km10³ Nelson Mountain Tuff vented at 26.9 Ma from an "underfit" caldera 30 km to the SW. New Ar-Ar single-crystal age determinations on sanidine have been critical to these reinterpretations.