A revised Ordovician age for the Sisargas orthogneiss, Galicia (Spain). Zircon U-Pb ion-microprobe and LA-ICPMS dating

Abstract

The Sisargas orthogneiss is located in the Schistose Domain of the Galicia Trás-Os-Montes Zone. It has previously been dated by Allegret and Iglesias (1987) who reported an U-Pb upper intercept at 570 ± 14 Ma, considered to be the crystallization age. For this reason, the Sisargas orthogneiss has traditionally been considered one of the oldest magmatic rocks of the Central Iberian Zone. However, new SIMS and LA-ICPMS data reveal that it crystallized at 479 ± 7 Ma and contains zircons with abundant pre-magmatic cores with ages clustering at 585 ± 15 Ma. This suggests that the Sisargas orthogneiss derives from the melting of a Pan-African protolith.