SMDS Dikes are approximately coeval with the start-up of PRB magmatic activity
A preliminary U/Pb zircon age of 120±1 Ma has been obtained from a 6-meter wide rhyolite dike in the densely intruded Rancho San Marcos region of the swarm (Figs. 2). A single fraction from an 8-meter wide rhyolite dike in the Tres Hermanos region ~60 km to the south displays virtually identical U/Pb systematics to the Rancho San Marcos sample although additional data is necessary to confirm this age.
The reconnaissance U/Pb data provides significant support to the idea that rhyolite dikes which dominate the swarm were emplaced in a relatively brief igneous episode. Interestingly, the preliminary 120±1 Ma zircon age from the dikes coincides closely with the inception of voluminous intrusive activity in the PRB which ranges from ~120-90 Ma (Silver and Chappell, 1988; Kimbrough et al., in review.).