K-Ar dates and field relations show that mid-Tertiary volcanic rocks usually assigned to the Datil Formation or Group in southwestern New Mexico can be interpreted as consisting of three major, slightly overlapping, volcanic cycles, which erupted about 28 to 38 + m.y., 23 to 28 m.y., and 20 to 23 m.y. B.P., respectively. Each cycle consists of lava flows, ranging in composition from andesite to rhyolite, and of voluminous silicic ash-flow tuff (ignimbrite) deposits, and is terminated by basaltic andesite and associated calc-alkalic rocks. Mid-Tertiary volcanism ceased at about 20 m.y. B.P., coincident with the beginning of Basin and Range faulting. Tholeiite and alkali basalt became the most common types of volcanic rocks after 20 m.y. B.P., although some felsic volcanism persisted.

The timing of mid-Tertiary events imposes constraints on plate-tectonic or other interpretations of volcanism and tectonism in western North America. Models designed to explain volcanism in the Great Basin could be expected to fit conditions in southwestern New Mexico also; the time of inception, type, variation trends, and volume of volcanism are essentially the same. The ∼20 m.y. date for the beginning of Basin and Range faulting is consistent with the constant-rate model of Atwater (1970). There is no evidence in New Mexico for progressive outward spreading of volcanism as described from the Great Basin by Armstrong and others (1969). As in the Great Basin, the mid-Tertiary rocks of southwestern New Mexico become progressively enriched in alkalis and depleted in calcium with time. This can be most clearly demonstrated by comparing rocks of pre-28 m.y. and post-28 m.y. age. In documenting a plate-tectonic model, Christiansen and Lipman (1972) and Lipman and others (1972) recognized a petrologic transition at about 28 m.y. They correlated it with the start of Basin and Range faulting, in conflict with the ∼20 m.y. date given here.

Porphyry-type mineralization is shown to be partly older than mid-Tertiary volcanism and partly contemporaneous with it.

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