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Eruptive and noneruptive calderas, northeastern San Juan Mountains, Colorado; where did the ignimbrites come from?

Peter W. Lipman and William C. McIntosh
Eruptive and noneruptive calderas, northeastern San Juan Mountains, Colorado; where did the ignimbrites come from?
Geological Society of America Bulletin (August 2008) 120 (7-8): 771-795


The northeastern San Juan Mountains, the least studied portion of this well-known segment of the Southern Rocky Mountains Volcanic Field are the site of several newly identified and reinterpreted ignimbrite calderas. These calderas document some unique eruptive features not described before from large volcanic systems elsewhere, as based on recent mapping, petrologic data, and a large array of newly determined high-precision, laser-fusion (super 40) Ar/ (super 39) Ar ages (140 samples). Tightly grouped sanidine ages document exceptionally brief durations of 50-100 k.y. or less for individual Oligocene caldera cycles; biotite ages are more variable and commonly as much as several hundred k.y. older than sanidine from the same volcanic unit. A previously unknown ignimbrite caldera at North Pass, along the Continental Divide in the Cochetopa Hills, was the source of the newly distinguished 32.25-Ma Saguache Creek Tuff (approximately 400-500 km (super 3) ). This regionally distinctive crystal-poor alkalic rhyolite helps fill an apparent gap in the southwestward migration from older explosive activity, from calderas along the N-S Sawatch locus in central Colorado (youngest, Bonanza Tuff at 33.2 Ma), to the culmination of Tertiary volcanism in the San Juan region, where large-volume ignimbrite eruptions started at ca. 29.5 Ma and peaked with the enormous Fish Canyon Tuff (5000 km (super 3) ) at 28.0 Ma. The entire North Pass cycle, including caldera-forming Saguache Creek Tuff, thick caldera-filling lavas, and a smaller volume late tuff sheet, is tightly bracketed at 32.25-32.17 Ma. No large ignimbrites were erupted in the interval 32-29 Ma, but a previously unmapped cluster of dacite-rhyolite lava flows and small tuffs, areally 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, marks a near-caldera-size silicic system active at 29.8 Ma. In contrast to the completely filled North Pass caldera that has little surviving topographic expression, no voluminous tuffs vented directly from the adjacent Cochetopa Park caldera, which is morphologically beautifully preserved. Instead, Cochetopa Park subsided passively as the >500 km (super 3) Nelson Mountain Tuff vented at 26.9 Ma from an "underfit" caldera (youngest of the San Luis complex) 30 km to the SW. Three separate regional ignimbrites were erupted sequentially from San Luis calderas within an interval of less than 50-100 k.y., a more rapid recurrence rate for large explosive eruptions than previously documented elsewhere. In eruptive processes, volcanic compositions, areal extent, duration of activity, and magmatic production rates and volumes, the Southern Rocky Mountains Volcanic Field represents present-day erosional remnants of a composite volcanic field, comparable to younger ignimbrite terranes of the Central Andes.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 120
Serial Issue: 7-8
Title: Eruptive and noneruptive calderas, northeastern San Juan Mountains, Colorado; where did the ignimbrites come from?
Affiliation: U. S. Geological Survey, Menlo Park, CA, United States
Pages: 771-795
Published: 200808
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 73
Accession Number: 2008-104163
Categories: Igneous and metamorphic petrologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2008085
Illustration Description: illus. incl. 4 tables, geol. sketch maps
N38°00'00" - N38°30'00", W107°00'00" - W106°00'00"
Secondary Affiliation: New Mexico Bureau of Geology and Mineral Resources, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200836
Program Name: USGSOPNon-USGS publications with USGS authors
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