PETROLOGY AND GEOCHEMISTRY OF THE MINERAL CREEK AND LAST CHANCE ANDESITES, WESTERN MOGOLLON-DATIL VOLCANIC FIELD, SOUTHERN NEW MEXICO

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. These andesites are monotonous medium-K, calc-alkaline rocks with 52-58 wt.% SiO₂ and display similar major element abundances. Trace element (Sr= 340-1021 ppm; Rb= 15-111 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr= 0.70495-0.70994) suggest three unique populations characterizing the two andesite formations: a high Sr concentration, low ⁸⁷Sr/⁸⁶Sr ratio group of andesite representing the Last Chance Andesite, (2) a low Sr concentration, high ⁸⁷Sr/⁸⁶Sr ratio andesites and (3) a low Sr concentration, low ⁸⁷Sr/⁸⁶Sr ratio basaltic andesites both representing the Mineral Creek andesite. 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. Locally this unit is 250 m thick. 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. Rare xenocrysts of quartz occur in the andesites. Lava units are of variable thickness with scoriaceous and amygdaloidal tops and bases. Flows have an aggregate thickness of approximately 300 m.

Here we present new major and trace element data combined with mineral textural and field data to distinguish between these units. 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. We suggest that these units represent the transition from bimodal to intermediate magmatism related to the end of subduction-related arc volcanism in the western MDVF.