INTERMEDIATE VOLCANIC ROCKS OF THE WESTERN MOGOLLON-DATIL VOLCANIC FIELD, SOUTHERN NEW MEXICO: A CHANGE IN MAGMATIC SOURCE AFTER IGNIMBRITE FLARE-UP

The Mineral Creek and Last Chance andesites located in the western Mogollon Datil Volcanic Field (MDVF) of southern New Mexico are indistinguishable in hand sample. These units are regionally extensive throughout the Mogollon Mountains and are some of the stratigraphically youngest intermediate composition volcanic rocks in the MDVF. The Mineral Creek andesite has been dated at 25.0 +0.5 Ma. It is reddish brown to gray crystal-poor andesite and basaltic andesite lava flows with interbedded mudflows and alluvial sandstones at the base of the unit. Lava flows contain a few percent phenocrysts of plagioclase, CPX and rare olivine. The Last Chance andesite has been dated between 25.0 to 23.2 Ma. Basaltic andesite and andesite lava flows and breccias are crystal-poor but contain approximately 5% phenocrysts including flow aligned plagioclase (An₃₀-An₅₀) > oxides + olivine and CPX.

Here we present new major and trace element data combined with mineral textural and field data to distinguish between these units for 11 whole rock samples. We also present a working geochemical model to decipher the relationship of the two units to one another and how the units are related to bimodal magmatism associated with the Mogollon Datil ignimbrite flare-up. Both andesites are monotonous medium-K, calc-alkaline rocks with 51.0-62.7 wt.% SiO₂, FeO_T from 4.7 to 10.9 wt.% and display similar major element abundances. Trace element contents are variable between the units (Sr= 290-787 ppm; Rb= 36-238 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr= 0.70495-0.70994) suggest at least two unique sources. Eu anomalies (0.91-.083) of all samples suggest plagioclase fractionation in a variable garnet stable source. The Mineral Creek Andesite is higher in SiO₂ content (SiO₂= 58.5-62.7 wt.%), lower FeO_T (4.7-8.9 wt.%) and higher Rb/Sr ratio (0.17-0.68) than the Last Chance Andesite (SiO₂= 51.0-53.2 wt.%; FeO_T= 9.16-10.9 wt.%, Rb/Sr= 0.05-0.08). Sr/Y ratios versus Y concentration and Rb/Sr ratios versus Sr concentration suggest that two geochemically distinct sources for the Last Chance and Mineral Creek andesites. The Mineral Creek Andesite is sourced from a garnet stable, plagioclase-free unstable source, while the Last Chance Andesite is sourced from a plagioclase stable, garnet free source.