Irregular bodies of non-welded Amalia Tuff within the Pena Tank Rhyolite, western San Luis Basin, north-central NM
William C. McIntosh, Daniel J. Koning, and Matt Zimmerer


The Peña Tank Rhyolite flow complex (new name) in north-central New Mexico consists of red-gray rhyolite lava that contains numerous, irregular bodies of white rhyolite, ranging from 3 m to 400 m in diameter. The two lithologies show outcrop-scale textures similar to those seen in co-mingled magmas, with fragments of red-gray lava included in the white rhyolite and veins of white rhyolite penetrating the red-gray lava. Although the two lithologies resemble intermingled lavas, thin-section observations unequivocally show that the white rhyolite consists of broken crystals in a matrix of glassy fragments. In contrast, the red-gray rhyolite consists of unbroken crystals surrounded by flow-banded, glassy to microcrystalline groundmass. Ar-Ar dating indicates that sanidine crystals in the red-gray Peña Tank rhyolite are 22.58 ± 0.06 Ma, which is 2.6 million years younger than the 25.21 ± 0.10 Ma sanidines of the white rhyolite. The texture, mineralogy, and ages of the two rhyolite units indicates they are not intermingled lava flows, but rather that the white rhyolite originated as Amalia Tuff erupted from the Latir caldera at 25.39 ± 0.04 Ma (Zimmerer and McIntosh, in review). The slight discrepancy in sanidine age between Amalia Tuff sanidine and the Peña Tank white rhyolite may reflect slight 40Ar loss related to reheating by the red-gray rhyolite. The unusual association of rhyolite lava containing bodies of older, non-welded tuff may have formed when Peña Tank Rhyolite lava erupted beneath a subaerial sheet of non-welded Amalia Tuff, incorporating large blocks of the ignimbrite. Fluidization of damp, non-lithified tuff allowed lava fragments to be mixed into the tuff and wispy tendrils of the tuff to penetrate the lava.


  1. McIntosh, William C.; Koning, Daniel J.; Zimmerer, Matt, 2011, Irregular bodies of non-welded Amalia Tuff within the Pena Tank Rhyolite, western San Luis Basin, north-central NM, in: Geology of the Tusas Mountains and Ojo Caliente Area, Koning, Daniel J.; Karlstrom, Karl E.; Kelley, Shari A.; Lueth, Virgil W.; Aby, Scott B., New Mexico Geological Society, Guidebook, 62nd Field Conference, pp. 223-234.

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