Exhumation history of the Variscan orogen in western Iberia as inferred from new K-Ar and 40Ar/39Ar data on granites from Portugal

• Published in: Tectonophysics 2021-08, Vol.812, p.228863, Article 228863
• Main Authors: Hildenbrand, A., Marques, F.O., Quidelleur, X., Noronha, F.
• Format: Article
• Language: English
• Subjects: 40Ar/39Ar and K-Ar dating; 40Ar/39Ar thermochronology; Argon; Batholiths; Biotite; Breccia; Carboniferous; Collapse; Collapse stage; Compression; Compression zone; Cooling; Crystallization; Deformation; Denudation; Exhumation; Feldspars; Geochronometry; Long-term exhumation; Mica; Micas; Mountains; Multi-diffusion domain approach (MDD); Muscovite; Orogeny; Permian; Quartz; Radiometric dating; Sciences of the Universe; Shear zone; Stretching; Tectonics; Tungsten; Uplift; Variscan orogeny; Western Iberia; Zircon
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2021.228863

Exhumation of the roots of collapsing orogens is a key process in the evolution of mountain belts, which is critical for the understanding of orogenic cycles. From new structural analyses, K-Ar and 40Ar/39Ar on muscovite, biotite and K-feldspar, and available U-Pb ages, we constrain the cooling and exhumation history of granitic batholiths from the main structural zones of the Variscan orogen in Iberia. We show that: (1) the oldest Ar dated granites (ca. 335 Ma on biotite) record exhumation of gneisses crystallized much earlier (up to ca. 530‐510 Ma U-Pb age); (2) granitoids crystallized at ca. 330 Ma record mostly ductile stretching (L-tectonites striking around N-S), consistent with N-S extension (Variscan intra-orogenic collapse); (3) most granites crystallized between ca. 320 and 305 Ma record ductile stretching consistent with deformation along NW-SE to ENE-WSW shear zones during the late-Variscan compression (C3); (4) granites crystallized after 305 Ma record mostly isotropic strain (no visible ductile foliation or lineation), consistent with their emplacement during/after the final collapse of the Variscan orogeny (E2); (5) comparison between Ar ages on micas and zircon U-Pb ages shows two contrasting situations: (i) similar ages, reflecting crystallization and fast cooling, which can be explained by relatively shallow intrusion and/or fast tectonic uplift; (ii) significant difference between U-Pb and Ar ages, supporting crystallization of deeper seated intrusions and their subsequent uplift for several Ma to tens of Ma; (6) thermochronological modelling on K-feldspar supports significant tectonic exhumation in the core of the chain between ca. 315 and 285 Ma, followed by late-stage passive denudation; 7) Late Carboniferous to early Permian tectonic exhumation was accompanied by significant formation of ore-deposits. Tin deposits occur in pegmatites and/or quartz veins associated with muscovite-rich syn-C3 granites and those of tungsten in quartz veins, breccia pipes and skarns associated with the E2 biotite-rich granites. [Display omitted] •Exhumation history of the Variscan orogen in Iberia from new K-Ar and 40Ar/39Ar data•K-Ar, 40Ar/39Ar and U-Pb record fast cooling/uplift during intra-orogenic collapse.•Tectonic exhumation in core of the chain (315-285 Ma) followed by passive denudation