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  • 21 Aug, 2019

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San Juan Volcanic Field
The San Juan volcanic field is part of the San Juan Mountains in southwestern Colorado. It consists mainly of volcanic rocks that form the largest remnant of a major composite volcanic field that covered most of the southern Rocky Mountains in the Middle Tertiary geologic time. There are approximately fifteen calderas known in the San Juan Volcanic Fields; however, it is possible that there are two or even three more in the region.

The region began with many composite volcanoes that became active between 35 and 40 million years ago, with peak activity in the time period around 35-30 million years ago. Around this time the activity began to include explosive ash-flow eruptions. Many of these volcanoes experienced caldera collapse, resulting in the fifteen to eighteen caldera volcanoes in the region today.

Phases of Volcanism

The San Juan volcanic field experienced two phases of volcanism.

The earlier volcanism took place during the Oligocene age of the Paleogene Period. It produced largely intermediate composition lavas and breccias, together with ash flow tuffs reflecting differentiation of the original magma. The precaldera intermediate volcanic rocks include the Conejos Formation in the southeastern part of the field. Intermediate volcanism did not cease with caldera eruptions, and included such voluminous intermediate volcanic rocks as the Huerto Andesite.

The later volcanism took place from Miocene to Pliocene in ages of the Neogene Period. It was characterized by bimodal volcanism, producing both low-silica alkaline flows and high-silica rhyolites. It is usually interpreted as a partial melt of the lower crust that was erupted onto the surface.

Cochetopa Dome (Cochetopa Caldera).
Aerial of Colorado State Highway 114 near Cochetopa Dome and Cochetopa Hills.

Notable calderas

Name Associated tuff Coordinates Age
Lake City Sunshine Peak Tuff 38°02′N 107°23′W / 38.03°N 107.38°W / 38.03; -107.38 22.93 ± 0.02 Ma
Creede Caldera Snowshoe Mountain Tuff 37°46′N 106°56′W / 37.76°N 106.94°W / 37.76; -106.94 26.5 ± 0.3 Ma
Cochetopa Nelson Mountain Tuff 38°12′N 106°45′W / 38.2°N 106.75°W / 38.2; -106.75 26.4 ± 0.5 Ma
San Luis Cebolla Creek Tuff
Rat Creek Tuff		 38°01′N 107°58′W / 38.01°N 107.97°W / 38.01; -107.97 26.9 Ma
South River Watson Park Tuff 37°40′N 106°56′W / 37.67°N 106.94°W / 37.67; -106.94 27.4? Ma
Bachelor Carpenter Ridge Tuff 37°49′N 106°55′W / 37.82°N 106.91°W / 37.82; -106.91 27.45 ± 0.05 Ma
La Garita Fish Canyon Tuff 37°46′N 106°56′W / 37.76°N 106.94°W / 37.76; -106.94 27.45 ± 0.05 Ma
Uncompahgre-San Juan Sapinero Mesa Tuff 37°58′N 107°32′W / 37.96°N 107.53°W / 37.96; -107.53 27.5 ± 0.5 Ma
Platoro Treasure Mountain Group 37°21′N 106°32′W / 37.35°N 106.53°W / 37.35; -106.53 28.4 Ma
Unknown Masonic Park Tuff 37°42′N 106°41′W / 37.70°N 106.69°W / 37.70; -106.69 28.7 Ma
North Pass Saguache Creek Tuff 38°12′N 106°32′W / 38.20°N 106.54°W / 38.20; -106.54 32.2 Ma
Bonanza Bonanza Tuff 38°19′N 106°05′W / 38.32°N 106.08°W / 38.32; -106.08 33.12 ± 0.03 Ma
Marshall Thorn Ranch Tuff 38°20′N 106°14′W / 38.34°N 106.23°W / 38.34; -106.23 33.6 ± 0.3 Ma
Chalcopyrite−quartz rock specimen, from Idarado Mine in San Juan Volcanic Field.

Economic Impact

The San Juan volcanic field has been a historically important mining district, producing lead, zinc, copper, gold, and silver. The ores were mostly deposited in and near calderas that experienced significant postcaldera activity. Ore veins were concentrated in fractures associated with caldera activity and in postcaldera intrusive bodies. The greatest mineralization took place near the youngest and most silicic intrusions of each caldera cycle.

See also

References

  1. ^ Steven, Thomas A.; Lipman, Peter W. (1976). "Calderas of the San Juan Volcanic Field, Southwestern Colorado". U.S. Geological Survey Professional Papers. Washington, DC: U.S. Government Printing Office. 958: 1–35. Retrieved 2012-05-16.
  2. ^ Lipman, Peter W.; Steven, Thomas A.; Mehnert, Harald H. (1970-08-01). "Volcanic History of the San Juan Mountains, Colorado, as Indicated by Potassium–Argon Dating". Geological Society of America Bulletin. 81 (8): 2329–2352. Bibcode:1970GSAB...81.2329L. doi:10.1130/0016-7606(1970)81[2329:VHOTSJ]2.0.CO;2. ISSN 0016-7606.
  3. ^ Lipman, P.W.; Doe, B.R.; Hedge, C.E.; Steven, T.A. (1978). "Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence". Geological Society of America Bulletin. 89 (1): 59–82. Bibcode:1978GSAB...89...59L. doi:10.1130/0016-7606(1978)89<59:PEOTSJ>2.0.CO;2. ISSN 0016-7606.
  4. ^ Colucci, M. T.; Dungan, M. A.; Ferguson, K. M.; Lipman, P. W.; Moorbath, S. (30 July 1991). "Precaldera lavas of the southeast San Juan Volcanic Field: Parent magmas and crustal interactions". Journal of Geophysical Research: Solid Earth. 96 (B8): 13413–13434. doi:10.1029/91JB00282.
  5. ^ Parat, Fleurice; Dungan, Michael A.; Lipman, Peter W. (1 May 2005). "Contemporaneous Trachyandesitic and Calc-alkaline Volcanism of the Huerto Andesite, San Juan Volcanic Field, Colorado, USA" (PDF). Journal of Petrology. 46 (5): 859–891. doi:10.1093/petrology/egi003.
  6. ^ Kennedy, Ben; Wilcock, Jack; Stix, John (October 2012). "Caldera resurgence during magma replenishment and rejuvenation at Valles and Lake City calderas". Bulletin of Volcanology. 74 (8): 1833–1847. doi:10.1007/s00445-012-0641-x.
  7. ^ Robinson, Joel E.; Dillon R. Dutton; David W. Ramsey; Peter W. Lipman; Tracey J. Felger (2006). Geologic Map of the Central San Juan Caldera Cluster, Southwestern Colorado: Geologic Investigations Series. Vol. I-2799. U.S. Geological Survey. Retrieved 2010-05-03.
  8. ^ Lipman, Peter W.; McIntosh, William C.; Zimmerer, Matthew J. (2013). "From ignimbrite to batholith, northeastern San Juan Mountains, ColoradoBonanza, Cochetopa Park, and North Pass calderas". Classic Concepts and New DirectionsExploring 125 Years of GSA Discoveries in the Rocky Mountain Region. doi:10.1130/2013.0033(14).
  9. ^ Lipman, Peter W.; Dungan, Michael A.; Brown, Laurie L.; Deino, Alan (1 August 1996). "Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs". GSA Bulletin. 108 (8): 1039–1055. doi:10.1130/0016-7606(1996)108<1039:REASAT>2.3.CO;2.
  10. ^ Sliwinski, J. T.; Bachmann, O.; Dungan, M. A.; Huber, C.; Deering, C. D.; Lipman, P. W.; Martin, L. H. J.; Liebske, C. (May 2017). "Rapid pre-eruptive thermal rejuvenation in a large silicic magma body: the case of the Masonic Park Tuff, Southern Rocky Mountain volcanic field, CO, USA". Contributions to Mineralogy and Petrology. 172 (5): 30. doi:10.1007/s00410-017-1351-3. hdl:20.500.11850/216533.
  11. ^ Varga, Robert J.; Smith, Brian M. (1984). "Evolution of the Early Oligocene Bonanza Caldera, northeast San Juan Volcanic Field, Colorado". Journal of Geophysical Research. 89 (B10): 8679. doi:10.1029/JB089iB10p08679.
  12. ^ Lipman, Peter W.; Zimmerer, Matthew J.; McIntosh, William J. (2015-12-01). "An ignimbrite caldera from the bottom up: Exhumed floor and fill of the resurgent Bonanza caldera, Southern Rocky Mountain volcanic field, Colorado". Geosphere. 11 (6): 1902–1947. Bibcode:2015Geosp..11.1902L. doi:10.1130/GES01184.1.
  13. ^ Steven, Thomas A.; Luedke, Robert G.; Lipman, Peter W. (1974). "Relation of mineralization to calderas in the San Juan volcanic field, southwestern Colorado". J. Res. US Geol. Surv. 2: 405–409.
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