Abstract

The Lucero area is a region of isolated, lava-capped mesas and buttes located in the transition zone of the southeastern Colorado Plateau adjacent to the central Rio Grande rift. Basaltic volcanism (basanites, alkali-olivine basalts, tholeiites, and evolved alkalic basaltic rocks) in the area began 8.3 m.y. ago and continued into Quaternary time, coinciding with much of the classic Basin and Range deformation and with the latest phase of rifting. Volcanism occurred in 3 distinct pulses 8.3−6.2, 4.3−3.3, and 1.1−0 m.y. ago.

Compositions of rocks erupted during each cycle comprise distinctly different, although partially overlapping, populations. They define a trend (from oldest to youngest) toward relatively larger volumes of tholeiites and an increasingly bimodal distribution with respect to alkalis versus silica. The alkaline basalts could have been derived from significantly deeper (50–70 km) than were the tholeiites (40–50 km) and thus suggest that the depth from which magmas were derived became shallower with time. Alternatively, the alkaline magmas may have been derived from the same depths as were the tholeiites by smaller amounts of partial melting. In either case, the relatively large volumes of tholeiites erupted during the intermediate and youngest cycles and the (relatively) low alkali content of the youngest tholeiites require an increase of melting at shallow mantle depths through time beginning ∼8 m.y. ago, accompanying the late Cenozoic reactivation of rifting.

A lull in magmatism from ∼7 to 4 m.y. ago between the oldest and intermediate cycles was widespread on the southeastern Colorado Plateau and in the central Rio Grande rift, although not on the Great Plains. A second period of quiescence 3−1 m.y. ago was of only local importance, correlating with a shift in the locus of volcanism to the Mount Taylor area. Such shifts in volcanism probably occur in response to local stresses in the lithosphere in an overall extensional stress field.

The temporal and compositional patterns that we observe in the Lucero area, although not yet understood, will provide constraints on models of rift evolution and guide future studies of volcanic rocks in this region.

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