40Ar/39Ar dating, sanidine chemistry and potential sources of the Las Tablas Tuff, northern New Mexico
Matthew J. Zimmerer and William C. McIntosh
The age and geochemistry of sanidine from the Las Tablas tuff was determined to investigate the source of this unit. A total of thirty Las Tablas tuff sanidine crystals from two geographically different locations were analyzed using the single-crystal laser-fusion 40Ar/39Ar dating technique. Two samples of the Las Tablas tuff yielded ages of 28.27±0.05 and 28.29±0.05 Ma. The weighted mean age of these samples is 28.28±0.07 Ma. The Las Tablas tuff is indistinguishable in age to two regional ignimbrites. The 28.201±0.046 Ma Fish Canyon Tuff was emplaced during the formation of the La Garita Caldera in the central San Juan volcanic field. The 28.22±0.05 Ma Tuff of Tetilla Peak was erupted during precaldera volcanism related to the Questa caldera in north-central New Mexico. Electron microprobe analyses were conducted to geochemically characterize sanidine from the three ignimbrites to determine if the Las Tablas tuff is correlative to either the distal outflow of the Fish Canyon Tuff, the Tuff of Tetilla Peak, or neither. Las Tablas tuff sanidine crystals exhibit oscillatory zoning, which precludes a correlation to the nonzoned sanidine of the Tetilla Peak Tuff. Though sanidine in the Fish Canyon Tuff are also zoned, there are subtle differences in Na2O and K2O content between the Fish Canyon Tuff and Las Tablas tuff. This prevents a simple correlation and suggests previously unrecognized geochemical variation of the Fish Canyon Tuff. Preliminary geochronology and geochemistry of the Las Tablas tuff indicate that this unit may represent distal outflow of the Fish Canyon Tuff or is related to a source that has yet to be identified.
- Zimmerer, Matthew J.; McIntosh, William C., 2011, 40Ar/39Ar dating, sanidine chemistry and potential sources of the Las Tablas Tuff, northern New Mexico, in: Geology of the Tusas Mountains and Ojo Caliente Area, Koning, Daniel J.; Karlstrom, Karl E.; Kelley, Shari A.; Lueth, Virgil W.; Aby, Scott B., New Mexico Geological Society, Guidebook, 62nd Field Conference, pp. 215-222.