Published K-Ar ages of laccoliths of the Henry and La Sal Mountains range from 23 to 56 Ma, with a bimodal distribution in the Eocene and late Oligocene epochs. Eocene ages require a period of magmatism during the Laramide Orogeny, a time of little or no igneous activity in the western United States at these latitudes. On the basis of 40Ar/39Ar and fission-track ages, we conclude that intrusive activity occurred during middle to late Oligocene and early Miocene time (31-20 Ma) in the Henry, La Sal, and Abajo Mountains. These results imply a genetic link to magmatism in the Reno-Marysvale and San Juan volcanic fields, which lie to the west and east of the Colorado Plateau, respectively. We interpret the small volume of the laccoliths, relative to the surrounding volcanic fields, to reflect differences in crustal composition and tectonic history of the plateau and adjacent areas.

Eocene K-Ar ages result from excess 40Ar or contamination by xenocrysts. Excess argon is manifested by trapped argon with 40Ar/36Ar ratios greater than atmosphere (295.5), resulting in total-gas or apparent K-Ar ages that are too old. Laser-fusion experiments and microprobe data show that some samples with Eocene or older total-gas ages and uncorrelated inverse correlation plots are affected by radiogenic argon residing in incompletely outgassed hornblende xenocrysts. All such samples contain middle- to late-Oligocene magmatic hornblende crystals as revealed by laser-fusion experiments. Xenocrysts may be derived from amphibolite xenoliths common in these rocks. Mixing calculations indicate that only 1%-4% contamination may be required to increase K-Ar ages by 10-15 Ma for xenocrysts outgassed as much as 75%. The 40Ar/39Ar method is capable of identifying small degrees of magmatic contamination by older K-bearing phases.

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