Early Variscan (Visean) granites in the core of central Pyrenean gneiss domes: implications from laser ablation U-Pb and Th-Pb studies

• Evidence for Cadomian magmatism (ca. 545; Ediacaran) in the Aston dome.
• Early Variscan granites (ca. 339 Ma; Visean) in the Bossòst and western Aston dome.
• Timing of Variscan LP-HT metamorphism and extensional D2 deformation in central Axial Zone.

Laser ablation U-Pb and Th-Pb single zircon and monazite geochronology was applied to seven samples from two Variscan gneiss domes in the central Pyrenean Axial Zone. Detrital zircon ages obtained from a paragneiss in the western Aston dome range from Neoarchean (ca. 2.57 Ga) to Late Neoproterozoic (ca. 560 Ma), suggesting that late Ediacaran sediments were deposited along the northern margin of Gondwana. Cadomian magmatism is recorded for the first time in the central Pyrenees from a biotite metagranite (ca. 545 Ma) in the central Aston dome. A second sample of an orthogneiss from the Aston dome yields a Middle Ordovician protolith age (ca. 467 Ma), similar in age to other dated orthogneisses of the Axial Zone. Three samples from granite stocks situated in the core of the Aston and Bossòst domes have crystallization ages of ca. 339–337 Ma, providing the first evidence of Visean igneous activity in the Axial Zone. In contrast to the larger and significantly younger (ca. 312–305 Ma) granite plutons along the margins of the gneiss domes, Visean magmatism caused LP-HT contact metamorphism (M2). Migmatization throughout the Aston dome, though less well-constrained by overgrowth ages on zircon (ca. 331 Ma) and monazite ages (ca. 340–320 Ma), is likely to be related to Visean intrusions. The main S2 schistosity in the Aston and Bossòst domes is coeval with the Visean intrusions. Inclusion trails in porphyroblasts nucleating in the contact aureoles document the continuous development of S2 related to local extension. Crystal-plastic deformation of the granites indicates that the D2 deformation event continued after their crystallization. Visean magmatism played a crucial role in early stages of dome formation by thermally weakening the metasedimentary mantle, and to a certain degree the orthogneiss as well, enhancing the development of orogen-parallel shear zones in an overall N–S compressive setting.

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