VOLUMINOUS POSTCALDERA ANDESITIC VOLCANISM ASSOCIATED WITH THE PLATORO CALDERA, SAN JUAN VOLCANIC LOCUS, COLORADO
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, An28-84 for the Summitville Andesite and An34-74 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.