Magma mixing as a petrogenetic process in the development of the Taos Plateau volcanic field, New Mexico

Abstract

The package of lavas exposed in the Red River gorge of the Taos Plateau volcanic field, northern New Mexico, consists of hybrids produced by simple two‐component magma mixing. The sequence of flows, which is graded compositionally from basalt at the base of the section to dacite at the top, represents the outpouring of progressively more silicic hybrids during the mixing process. The end‐members were basalt (50% SiO2) and dacite only slightly more silicic than the uppermost dacite in the gorge (61–62% SiO2). Hybrids (52–57% SiO2) are characterized by large skeletal olivine phenocrysts and exhibit linear chemical variations between end‐member compositions for all major and trace elements. Major and trace element mass balance calculations concur on the proportions of end‐members necessary to form each hybrid; chemical models limit the amount of subsequent olivine fractionation to less than 5%. The presence of groundmass olivine in andesitic hybrids implies that eruption occurred shortly after, or during, mixing. Estimated density and viscosity contrasts between the end‐members would appear to inhibit mixing of basaltic and dacitic magmas, unless basalt were forcefully injected into a dacitic magma chamber. Chemical and petrographic similarities between other olivine andesites and two‐pyroxene dacites of the Taos Plateau volcanic field and the Red River gorge hybrids suggest that magma mixing was an important process in the Taos Plateau magmatic system.