New Mexico Geological Society Annual Spring Meeting — Abstracts

The Organ caldera in south-central New Mexico formed during eruptions ~36 Ma, and the volcanic deposits and plutonic bodies are now exposed in the Organ Mountains. This research project provides the first analyses of volatile contents and trace and major element concentrations of melt inclusions, and crystallization pressures from the first and last erupted caldera units, the Cueva Tuff and the Squaw Mountain Tuff ignimbrites. Through geochemical analyses of melt inclusions, whole rocks and minerals, information has been obtained regarding pre-eruptive characteristics of the magma chamber including chamber stratification, volatile contents preceding the eruptions, and the volume of the eruptible magma chamber. Previous studies suggested that the three caldera-forming eruptions resulted from a single, stratified magma chamber being repeatedly tapped, and whole-rock geochemistry supports this claim (Seager and McCurry, 1988). However, results thus far show variable volatile contents (~2.2-6.8% H2O) and a narrow range of major elements from the first-erupted Cueva Tuff melt inclusions, suggesting a more homogeneous, convecting magma chamber at depths between ~2 and 10 km. Whole rock analyses from this and previous studies show similar major element composition for the Cueva Tuff. Analyses of volatile contents from melt inclusions from the last-erupted unit, Squaw Mountain Tuff, have been inconclusive, but major element compositions are more heterogeneous than the Cueva Tuff. Both melt inclusion and XRF whole rock data from the Squaw Mountain Tuff span between the Cueva Tuff and West Side Lavas (eruptive unit following the Squaw Mountain) compositions. These data currently suggest the possibility that the injection of the West Side Lavas and the resultant mixing with residual Cueva Tuff helped initiate the eruption of Squaw Mountain Tuff. This hypothesis is further supported by magma mixing or mingling observed in samples of the upper Squaw Mountain Tuff. This hypothesis is being tested through a detailed investigation of mineral chemistry, textures and zoning, which should reveal any mixing prior to eruption. Additionally, a secondary estimate of magma H2O and storage pressures from the Squaw Mountain Tuff is required, as the melt inclusions in Squaw Mountain underwent volatile loss during rehomogenization, resulting in unreliable data. I will use a published hygrometer to determine melt H2O contents from Squaw Mountain Tuff feldspars to provide an estimate of pressures and depths of Squaw Mountain Tuff crystallization. The results of this project provide new insights into the formation and evolution of a large magma body, and processes that occurred prior to the eruptions of Organ caldera.

References:

1. Seager, W., 1981, Geology of Organ Mountains and southern San Andres Mountains, New Mexico: New Mexico Bureau of Mines & Mineral Resources, 47-52.
2. Seager, W., and McCurry, M., 1988, The cogenetic Organ cauldron and batholith, South Central New Mexico: Evolution of a large- volume ash flow cauldron and its source magma chamber: Journal of Geophysical Research, v. 93, p. 4421-4433.