A PETROCHEMICAL TEST OF COMPETING IDEAS ON THE EMPLACEMENT HISTORY OF SOUTH MOUNTAIN RHYOLITE, VALLES CALDERA, NEW MEXICO

South Mountain is one of eight post eruptive rhyolite dome complexes (1.23 Ma to 0.52 Ma) that were emplaced along ring fractures after formation of the c.a. 1.25 Ma Valles caldera. Early workers proposed that South Mountain (0.52 Ma) consisted of one eruptive phase, whereas more recent geologic mapping established four possible eruptive flows from the dome complex. A detailed field and petrochemical study of the South Mountain dome and flow complex was conducted to test these competing ideas.

The mineral assemblages of samples from the four different flows are similar. They contain phenocrysts of quartz, sanidine, albite-oligoclase, biotite, and minor oxyhornblende that are set in a glass matrix that varies from spherulitic to flow laminated. The variable textures of the groundmass in the four identified flows, possibly reflects different mechanisms of emplacement and cooling histories. Major and trace element geochemistry of these samples indicates that they are calk-alkaline, high-silica rhyolites. The similar mineral assemblages and geochemical signatures of all flow phases from the dome indicate a rather homogeneous magma composition. The four morphologically different flows can be explained as four different eruptive events from the same magma body. Additional major and trace element geochemical data are being applied to try and further understand the magmatic history of the flows, and to assess fractionation mechanisms.