Contemporaneous Trachyandesitic and Calc-alkaline Volcanism of the Huerto Andesite, San Juan Volcanic Field, Colorado, USA |
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Authors: | PARAT, FLEURICE DUNGAN, MICHAEL A. LIPMAN, PETER W. |
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Affiliation: | 1 DÉPT. MINÉRALOGIE, UNIVERSITÉ DE GENÈVE, RUE DES MARAÎCHERS 13, 1211 GENÈVE 4, SWITZERLAND 2 US GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CA 94025, USA |
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Abstract: | Locally, voluminous andesitic volcanism both preceded and followedlarge eruptions of silicic ash-flow tuff from many calderasin the San Juan volcanic field. The most voluminous post-collapselava suite of the central San Juan caldera cluster is the 28Ma Huerto Andesite, a diverse assemblage erupted from at least56 volcanic centres that were active around the southernmargins of the La Garita caldera shortly after eruption of theFish Canyon Tuff. These andesitic centres are inferred, in part,to represent eruptions of magma that ponded and differentiatedwithin the crust below the La Garita caldera, thereby providingthe thermal energy necessary for rejuvenation and remobilizationof the Fish Canyon magma body. The multiple Huerto eruptivecentres produced two magmatic series that differ in phenocrystmineralogy (hydrous vs anhydrous assemblages), whole-rock majorand trace element chemistry and isotopic compositions. Hornblende-bearinglavas from three volcanic centres located close to the southeasternmargin of the La Garita caldera (Eagle MountainFourmileCreek, West Fork of the San Juan River, Table Mountain) definea high-K calc-alkaline series (5765 wt % SiO2) that isoxidized, hydrous and sulphur rich. Trachyandesitic lavas fromwidely separated centres at Baldy MountainRed Lake (westernmargin), Sugarloaf Mountain (southern margin) and Ribbon Mesa(20 km east of the La Garita caldera) are mutually indistinguishable(5561 wt % SiO2); they are characterized by higher andmore variable concentrations of alkalis and many incompatibletrace elements (e.g. Zr, Nb, heavy rare earth elements), andthey contain anhydrous phenocryst assemblages (including olivine).These mildly alkaline magmas were less water rich and oxidizedthan the hornblende-bearing calc-alkaline suite. The same distinctionscharacterize the voluminous precaldera andesitic lavas of theConejos Formation, indicating that these contrasting suitesare long-term manifestations of San Juan volcanism. The favouredmodel for their origin involves contrasting ascent paths anddifferentiation histories through crustal columns with differentthermal and density gradients. Magmas ascending into the mainfocus of the La Garita caldera were impeded, and they evolvedat greater depths, retaining more of their primary volatileload. This model is supported by systematic differences in isotopiccompositions suggestive of crustmagma interactions withcontrasting lithologies. KEY WORDS: alkaline; calc-alkaline; petrogenesis; episodic magmatism; Fish Canyon system |
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Keywords: | : alkaline calc-alkaline petrogenesis episodic magmatism Fish Canyon system |
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