Exhuming a cold case: The early granodiorites of the northwest Iberian Variscan belt—A Visean magmatic flare-up?

Abstract

In this study we report laser ablation-inductively coupled plasma-mass spectrometer U-Pb ages of granitoids from the so-called early granodiorites of the northwest Iberian Variscan belt. The U-Pb results attest to significant magmatic activity in Visean time (ca. 347-337 Ma) that generated a hitherto poorly constrained granitoid suite in the northwest Iberian tract of the western European Variscan belt realm. This early Carboniferous suite (ECS) is mainly composed of peraluminous cold and hot crustal granodiorites and monzogranites with minor associated mafic rocks that attest to minor involvement of mantle melting. Based on the geochronological and geochemical data, we compare the Visean granitoids with younger Variscan granitoids in northwest Iberia and, in view of the tectonothermal scenarios of the Variscan collision in northwest Iberia, propose a model for the genesis of the ECS in northwest Iberia that involves rapid melting upon fast exhumation of the thickened Gondwanan crust in the course of the protracted Variscan collision.

The significance of early Permian and early Carboniferous U–Pb zircon ages in the Bossòst and Lys‐Caillaouas granitoids (Pyrenean Axial Zone)

“…Hence, a probable cause for the origin of early Carboniferous zircon in the Pyrenees is a magmatic event due to thermal relaxation, which produced small segregated igneous bodies and the local migmatization in deep zones of the crust during the Variscan collision. This is in line with what Gutiérrez‐Alonso et al () recently proposed for the adjacent Iberian (Variscan) Massif. The age of migmatization inferred by Mezger and Gerdes () in the Aston dome, between 340 and 320 Ma, agrees with this hypothesis.…”

“…Yet, the magmatic bodies that produced early Carboniferous zircon populations in the Pyrenees remain elusive. In addition, this contrasts to what is observed in the Central‐Iberian zone of the Variscan Iberian Massif, where the Visean magmatic activity and the related magmatic bodies are well constrained (e.g., Gutiérrez‐Alonso et al, ).…”

“…It should be also added that zircon and monazite grains with early Carboniferous ages (ca. 320 Ma), considered either as xenocryst (i.e., inherited crystals) or as syn‐magmatic, have been found in other metamorphic domes in the adjacent Variscan massifs (e.g., Gutiérrez‐Alonso, Fernández‐Suárez, López‐Carmona, & Gärtner, ; López‐Moro et al, ; Martínez et al, ; Martínez, Reche, & Iriondo, ; Poilvet, Poujol, Pitra, Van Den Driessche, & Paquette, ). A particularly well‐documented case is the Tormes Dome in the Iberian Massif, where the limited overprint of the widespread late Carboniferous–early Permian magmatism allowing to recognize several igneous rocks with an unequivocal early Carboniferous age (López‐Moro et al, ).…”

“…It should be also added that zircon and monazite grains with early Carboniferous ages (ca. 320 Ma), considered either as xenocryst (i.e., inherited crystals) or as syn-magmatic, have been found in other metamorphic domes in the adjacent Variscan massifs (e.g., Gutiérrez-Alonso, Fernández-Suárez, López-Carmona, & Gärtner, 2018;López-Moro et al, 2018;Martínez et al, 2016;Martínez, Reche, & Iriondo, 2008;Poilvet, Poujol, Pitra, Van Den Driessche, & Paquette, 2011 (ii) All the domes and related metamorphism formed contemporarily, and the geological meaning of one of the two different Variscan zircon ages is misinterpreted.…”

The enigmatic curvature of Central Iberia and its puzzling kinematics

“…(3) The Central Iberian Zone represents the Gondwanan hinterland with Barrovian and Buchan metamorphism and is intruded by igneous rocks of various ages (e.g. Macaya et al, 1991;Díez Balda, 1995;Gutiérrez-Alonso et al, 2018). (4) The Ossa-Morena Zone represents the most distal zone of the Gondwana platform, and is characterized by a metamorphic fold-and-thrust belt with dominantly sinistral displacement (e.g.…”