A revised Ordovician age for the Miranda do Douro orthogneiss, Portugal. Zircon U-Pb ion-microprobe and LA-ICPMS dating

Abstract

The Miranda do Douro orthogneiss was believed to be the oldest magmatic rock of the Central Iberian Zone, on the base of a U-Pb discordia upper intercept of 618 ± 9 Ma. Nevertheless, new ion-microprobe and LA-ICPMS U-Pb zircon dating revealed that the crystallization age was 483 ± 3 Ma. The orthogneiss also contains a 605 ± 13 Ma zircon population that indicates that the source-rock for the Ordovician magma was Pan-African. Moreover, a few ~3.17 Ga zircon grains were also recorded. These grains are the oldest found so far in Iberia, and its occurrence would suggest the involvement of an Archean crust in the Pan-African orogeny.

Keywords

• U-Pb dating
• Ollo de Sapo
• Pre-Variscan
• Central Iberian Zone
• Gneiss.