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ABSTRACT

The Mogollon-Datil volcanic field is a 40–24 Ma cluster of calderas that formed during ignimbrite flare-up eruptions in southern New Mexico associated with sub-duction, and possible delamination, of the Farallon plate beneath the North American plate. This study uses magmatic zircon sampled from four ignimbrites from a nested caldera system and an additional ignimbrite located outside of the nested system to compare the processes and timing of magma accumulation in southern New Mexico. These ignimbrites include: the Whitewater Tuff, the Cooney Canyon Tuff, the Davis Canyon Tuff, and the Shelley Peak Tuff from the Mogollon Mountains and the Bell Top 4 Tuff from the Uvas volcanic field. The ignimbrites range from crystal-poor, high-silica rhyolite to crystal-rich, low-silica rhyolite. We compare previous 40Ar/39Ar sanidine eruption ages to new U-Pb crystallization ages and trace-element compositions of zircon. Weighted mean zircon ages define two magmatic groups. Group one includes the Bell Top Tuff (34.5 ± 0.5 Ma), the Cooney Canyon Tuff (34.8 ± 0.8 Ma), and the Whitewater Creek Tuff (36.2 ± 0.4 Ma). The second group includes the Davis Canyon Tuff (28.7 ± 0.5 Ma) and the Shelley Peak Tuff (29.6 ± 0.5 Ma). Weighted mean zircon ages are within published 40Ar/39Ar ages, with the exception of the Shelley Peak Tuff, which is ~1 m.y. older. Hafnium contents and Th/U and Yb/Gd ratios suggest the dominant mechanism that produced eruptible melt was rejuvenation or remobilization of a crystal mush accompanied by minimal partial melting of the continental crust.

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