Isotopic Ages of Igneous Intrusions in Southeastern Utah: Evidence for a Mid-Cenozoic Reno-San Juan Magmatic Zone (original) (raw)

New fission-track ages for the Abajo, Henry, and La Sal Mountains in southeastern Utah show that these laccolithic centers were emplaced between about 30 and 20 Ma, making them contemporaneous with the Reno-Marysvale and San Juan volcanic zones to the west and east. Ranges of zircon ages for these intrusions are as follows: 22.6 ± 2.2 to 28.6 ± 3.4 Ma (Abajo Mountains); 20.0 ± 1.9 to 29.2 ± 2.3 Ma (Henry Mountains); 28.7 ± 2. 7 Ma (La Sal Mountains, one sample). These data suggest the existence of an essentially continuous, intracontinental magmatic zone extending from Reno to the San Juan Mountains during the Oligocene and early Miocene. The length of this zone (more than 1000 km) and its orientation perpendicular to the trend of the subduction zone along the western coast of North America are additional constraints on the subduction models generally used to explain mid-Cenozoic igneous activity in the western United States.