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Bulletin 135 — Paleomagnetism and 40Ar/39Ar ages of ignimbrites, Mogollon–Datil volcanic fields, southwestern New Mexico

By W. C. McIntosh, L. L. Kedzie, and J. F. Sutter, 1991, 79 pp., 3 tables, 32 figs., 3 appendices.

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This report presents 40Ar/39Ar sanidine ages and paleomagnetite data for 36 ignimbrites and associated lavas in the Eocene-Oligocene Mogollon-Datil volcanic field of southwestern NM. 40Ar/39Ar age spectra from the ignimbrites yield plateau ages that range from 36.2 to 24.3 Ma and show within-sample and within-unit 1s precision of ±0.5% or better. These ages agree closely with independently established stratigraphic order and indicate that Mogollon-Datil ignimbrite activity was highly episodic, being confined to four brief (<2.6 Ma) eruptive intervals separated by 1.5–3 Ma long hiatuses. Ignimbrite outflow sheets show stable paleomagnetic remanence directions that are uniform over most of their areal extents, including facies ranging from thick, densely welded proximal ignimbrites to unwelded distal fringes as thin as 1.5 m. Used in concert with lithologic and stratigraphic position data, 40Ar/39Ar ages and paleomagnetic directions allow accurate long-range ignimbrite correlations that provide reliable ties between previously established subregional stratigraphic sequences. These correlations have helped to establish an integrated time-stratigraphic framework for the Mogollon-Datil volcanic field.

Since the mid-1950s the 40,000 km2 late Eocene-Oligocene Mogollon-Datil volcanic field has been the focus of numerous mapping, stratigraphic, geochemical, petrologic, and isotopic studies, primarily by workers from NM universities, NM Bureau of Mines and Mineral Resources, and the U.S. Geological Survey. Detailed geologic mapping has been successfully used to unravel the stratigraphy in subregions of the field, but synthesis of subregional stratigraphic data has been frustrated by the inability to correlate ignimbrites over long distances. Correlations based solely on mapping have been limited by discontinuous basin-and-range outcrop patterns. Lithology and geochemistry, although useful as first-order correlation criteria, have proven unreliable on a regional scale. Many stratigraphically distinct units are lithologically similar, and individual units show lateral, vertical, and sectoral variations in mineralogy, chemistry, welding texture, and phenocryst concentration and size, analogous to variations shown by ignimbrites elsewhere. Conventional K-Ar dating techniques have been helpful in some cases, but lack the resolution required by most of the correlation problems.

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