Grizzly Peak cauldron, Colorado: structure and petrology of a deeply dissected resurgent ash-flow caldera
The 34-Ma-old Grizzly Peak cauldron is a deeply eroded 17- by 23-k caldera structure on the crest of the Sawatch Range in west-central Colorado. Subsidence of the cauldron along bounding ring faults resulted from eruption of the Grizzly Peak Tuff, which ponded in the caldera as it formed. An inner ring fracture zone divides the cauldron into two segments and is a growth fault in the intracaldera tuff. Following subsidence, the cauldron was uplifted to form a complexely faulted resurgent dome. Intracaldera Grizzly Peak Tuff, as thick as 2.7 km, is a single cooling unit zoned from high-silica rhyolite at the base to low-silica rhyolite at the eroded top and, further, contains dacite to mafic latite welded pumice clasts (fiamme) in two heterogeneous tuff horizons in the upper half of the unit. Tuff zoning defined by fiamme is unusually strong for a single volcanic unit: 77 to 57% SiO2. Major-element trends can be modeled by crystal fractionation using observed phenocrysts. Inflections in the trends of Zr, Hf, Th, REE, Y, Mn, and Sc correlated with changes in phenocryst mineralogy and composition, indicating control by crystal-liquid equilibria. Trends for some trace-element cannot be fit with the crystal-fractionation model; over different portions of the zonation, Zr, Hf, LREE, and Rb enrich at too high a rate, and Ta, Nb, and Ba enrich at too low a rate. Progressive batch melting of a single source can also be eliminated because Co, Cr, Eu, and Sr, and Ba are too strongly depleted over different silica intervals. The zonation must therefore be the result of a combination of processes. Compositional trends defined by a series of intracaldera intrusions can be explained by hybridization, during resurgence, of the unerupted portion of the zoned magma column sampled in the tuff eruption.
- Research Organization:
- Stanford Univ., CA (USA)
- OSTI ID:
- 6409171
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580202* -- Geophysics-- Volcanology-- (1980-1989)
580300 -- Mineralogy
Petrology
& Rock Mechanics-- (-1989)
CHALCOGENIDES
COLORADO
CORRELATIONS
ELEMENTS
FEDERAL REGION VIII
GEOLOGIC FAULTS
GEOLOGIC FRACTURES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
GEOLOGY
GROUND SUBSIDENCE
METALS
MINERALOGY
NORTH AMERICA
OXIDES
OXYGEN COMPOUNDS
PETROLOGY
RARE EARTHS
SILICON COMPOUNDS
SILICON OXIDES
TRACE AMOUNTS
USA
VOLCANIC REGIONS