Part I. Age of three intrusive rocks in North-Central Mexico

Abstract

The ages of the three intrusive rocks discussed were obtained using the lead-alpha method or Larsen's method, with zircon contained in the rocks being used for isotopic determinations. The sample LG-3, from the rhyodacitic porphyry trunk of Cerro Redondo, yielded an age of 40 ± 10 million years, corresponding to the Late Eocene. The sample LG-14, from the monzonitic trunk of Rancho El Pico, gave an age of 80 ± 10 million years, equivalent to the Santonian. The determination from sample LG-20, from the rhyodacitic porphyry of Pichaguilla, was 70 ± 10 million years, with emplacement occurring during the Maastrichtian. These ages may correspond to the emplacement time of the cited bodies; however, other isotopic determinations using different methods are recommended to confirm them.

The age of 40 million years from the trunk of Cerro Redondo marks an intrusive event already confirmed with the determination of other ages, for which the name "Zacatecano Intrusive Event" is proposed. The ages of 80 million years and 70 million years from the trunks of Rancho El Pico and Pichaguilla indicate an era of magmatic activity known as the "Santaluciano Intrusive Event," corresponding to the anatectic phase of the Mexican geotectonic cycle.