Complex Paleozoic magmatic and metamorphic evolution in the Argentera Massif (Western Alps) resolved with U-Pb dating

Abstract

The timing of pre-Alpine magmatism and metamorphism in the Gesso-Stura Terrain (NE Argentera massif, External Crystalline Massifs) has been investigated by U-Pb dating. Conventional ID-TIMS analysis of zircons from an eclogite constrained the intrusion of the gabbroic protolith at 462 ± 6 Ma, in agreement with an ion microprobe age of 457 ± 6 Ma on the same sample. Regardless of their pristine magmatic zoning, the zircons from the eclogite suffered lead loss during one or more thermal events. A lower discordia intercept (323 ± 12 Ma) and the youngest zircon rim dated by ion-microprobe (330 ± 5 Ma) constrain the age of the last of these events, i.e. the Variscan amphibolite-facies metamorphism, to the Namurian. Zircon from a metadacite indicates that the acid magma crystallised during Late Ordovician at 443 ± 3 Ma. Ion microprobe dating of zircon from a monzonite affected by Variscan anatexis yielded an intrusion age of 332 ± 3 Ma. Our results help reconstructing the pre-Alpine evolution of the Gesso-Stura terrain of the Argentera basement, which records several magmatic and metamorphic events in the time span between Early Ordovician and Late Carboniferous. The zircon behaviour in the dated rocks bears some implications for U-Pb geochronology.