VOLUMINOUS POSTCALDERA ANDESITIC VOLCANISM ASSOCIATED WITH THE PLATORO CALDERA, SAN JUAN VOLCANIC LOCUS, COLORADO

Following the eruption of large volume ignimbrites, andesitic lavas filled the Platoro caldera and erupted from centers in and adjacent to the caldera. These include the Summitville Andesite and Sheep Mountain Andesite. The Summitville Andesite lavas, locally more than 500 m thick, comprise two members intercalated with ignimbrites of the Treasure Mountain Group. The lower member consists of lavas and volcaniclastic sedimentary rocks deposited after the ca. 29 Ma eruption of the La Jara Canyon Tuff, while the upper member which consists of lavas that were emplaced following the 28.8 Ma eruption of the Chiquito Peak Tuff. Upper Summitville Andesite is overlain by Sheep Mountain Andesite. New whole rock and trace element geochemistry coupled with mineral compositions enable comparison of these lavas to assess their role in the Platoro magmatic system.

Summitville Andesite ranges from 54-61 wt. % SiO₂ and is aphanitic to porphyritic with up to 20% phenocrysts of plagioclase, augite and orthopyroxene. The Sheep Mountain Andesite ranges from 56-61 wt. % SiO₂ and locally is porphyritic with up to 35% phenocrysts of plagioclase, augite and orthopyroxene. The lavas fall along a high-K calc-alkaline trend and have isotopic compositions similar to those of central San Juan postcaldera andesites. Plagioclase anorthite content in both units shows a broad range, An_28-84 for the Summitville Andesite and An_34-74 for the Sheep Mountain Andesite. This range is similar to that for the Chiquito Peak Tuff which contains high-An cores in plagioclase phenocrysts. Clinopyroxene phenocrysts from the Sheep Mountain Andesite are higher in Ca but overlap in Mg# (67-79) with those of the Summitville Andesite (62-77); both are broader than the narrow range of Mg# in the Chiquito Peak Tuff clinopyroxenes (73-76). The Platoro postcaldera andesitic lavas represent a substantial volume of magma that may have ponded beneath the Platoro caldera perhaps rejuvenating, mixing, and remobilizing the Chiquito Peak mush and were then subsequently erupted.