Abstract

The Rio Puerco volcanic necks and Mesa Prieta, located in the transition zone between the Rio Grande rift and southeastern Colorado Plateau, New Mexico, are part of the larger Mount Taylor volcanic field. The comparatively small volume, but numerous, eruptions represented by the volcanic necks provide geochronologic insight to the larger magmatic system at Mount Taylor and in a broader sense to regional late Cenozoic volcanism of the southeastern Colorado Plateau transition zone. Whole-rock 40Ar/39Ar ages from 17 basaltic plugs, flows, and dikes range in age from 4.49 to 2.05 Ma and overlap the 3.73 to 1.57 Ma volcanism observed at Mount Taylor.

Volcanic eruptions in the Rio Puerco Valley were few and sporadic prior to 3.17 Ma; after this time, eruptions progressively increased to a zenith at ca. 2.70 Ma and ceased altogether after 2.56 Ma. Volcanism at Mesa Prieta (2.36–2.05 Ma) is significantly younger than that recognized for the volcanic necks and is considerably more voluminous and chemically evolved. Paleomagnetic data from the volcanic necks show that basalts carry reliable thermoremanent magnetizations; 9 of 13 sites exhibit well-grouped statistical characteristics. Polarity results provide an independent check of the 40Ar/39Ar analytical accuracy and are useful in resolving problems associated with excess Ar and/or Ar loss.

The magmatic evolution of the Rio Puerco necks differs from the rest of the Mount Taylor volcanic field in duration of volcanism, erupted volume, and composition. These differences are believed to be related to the nonuniform character of the lithosphere's structure and dynamic thermal construct across the Jemez zone in the vicinity of the Mount Taylor volcanic field. Our model proposes a zone of localized high heat flow and higher magma production beneath Mount Taylor volcano from 3.73 to 1.57 Ma to produce a suite of voluminous basalt-rhyolite compositions. However, east of Mount Taylor in the Rio Puerco Valley where the lithosphere is proposed to be cooler and more rigid throughout the same time interval, small-volume, xenolith-bearing, mafic eruptions are more typical. A cooler lithosphere is consistent with both low magma production rates and the observed small-volume eruptions of the Rio Puerco necks. A preexisting structural trend in the Rio Puerco Valley is believed to have assisted the ascent of volatile-rich, Rio Puerco magmas, particularly through the upper crust.

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