THE PRE-ERUPTIVE PLUMBING SYSTEM OF INTERMEDIATE MAGMAS FROM THE MOGOLLON-DATIL VOLCANIC FIELD: INSIGHTS FROM PYROXENE AND PLAGIOCLASE MINERAL CHEMISTRY AND GEOTHERMOBAROMETRY

The Mollogon-Datil volcanic field in southern and central New Mexico is best known for caldera forming ignimbrite eruptions. Intermediate composition volcanic rocks are considered to only be prevalent early in the volcanic field’s history, however, they are found intermixed with rhyolite and are the dominant composition in some areas of the volcanic field. In southwestern New Mexico, large volume intermediate volcanic rocks of the Bearwallow Mountain Andesite and the Andesite of Dry Leggett are over 300 m thick in the type localities and can be found throughout the southwest New Mexico. The Bearwallow Mountain Andesite is a series of calc-alkaline andesite to basaltic andesite lava flows ranging in age from 23-27 Ma. Lava flows were erupted from at least four different vents. Lavas range from 48-63 wt% SiO₂ and from 5.2-10.5 wt% FeO. Basaltic andesites are amygdule-rich and crystal-poor with 1-5% phenocrysts of plagioclase, orthopyroxenes, and clinopyroxenes with trace olivine and/or biotite. Here, we present major and trace element data from plagioclase, orthopyroxenes and clinopyroxenes phenocrysts from two lava flows representing the top and bottom of the volcanic stratigraphy.

New mineral-melt thermobarometry and mineral chemistry data reveal variable crystallization conditions and enable a detailed reconstruction of the magma plumbing system. Clinopyroxene phenocrysts and microlites are diopsides and augites with En_42-46, Wo_42-45 and Mg# of 59-70. Orthopyroxenes are hypersphenes with En_64-81 and Mg# of 38-69. Core to rim variation for diopsides are homogeneous ranging from En_42-43 and Wo_43.5-44. Hypersphenes are more variable ranging from En_68-81 and Mg# of 49-50 within a single phenocryst. Pressures and temperatures of magma crystallization in the MDVF are poorly constrained, especially for intermediate composition systems. Two-pyroxene and clinopyroxne-liquid thermobarometry suggest magmatic temperatures of 900-1070°C and storage pressures of 440-800 MPa. We suggest that these rocks record the hybridization of the continental crust at 28-27 Ma, changing magma composition and eruption style from caldera forming rhyolite magmas to intermediate composition lava flows effectively ending explosive volcanism in the southwest Mogollon-Datil volcanic field.