New mapping and geochronology constraints on the Variscan plutonism, metamorphism and deformation in the Ossa-Morena Zone (SW Iberian Massif)

The western-central European Variscan orogen developed during the Upper Devonian to Carboniferous continental collision between Gondwana and Laurussia, culminating in the assembly of Pangea. This orogen records a complex sequence of tectono-thermal events providing insights into continental crustal evolution and lithospheric deformation mechanisms. Late Devonian D₁ contractional deformation is associated with crustal thickening, Mississippian D₂ extensional deformation was probably caused by gravitational collapse of the mountain range, and Pennsylvanian D₃ contractional deformation represents subsequent crustal shortening. The Iberian Massif, located at the core of the Variscan Orogen, offers exceptional conditions for detailed analysis of deep-to-shallow crustal Variscan tectono-thermal processes, preserving these three superimposed deformation events.

In the Ossa-Morena Zone (SW Iberian Massif), the youngest Variscan orogenic activity is associated with the emplacement of syn- to late-D₃ plutons. We present new geological mapping, whole-rock geochemistry, and zircon U-Pb geochronology from the Pennsylvanian Figueira e Barros-Ervedal and Fronteira plutons (west-central Ossa-Morena Zone). These shallow-crustal, calc-alkaline, peraluminous granodioritic to granitic intrusions are syn- to late-D₃ because they crosscut D₂-D₃. SHRIMP U-Pb zircon dating indicates crystallisation ages of 307 ± 3 Ma and 308 ± 2 Ma for the Figueira e Barros-Ervedal and Fronteira plutons, respectively.

Their host metamorphic succession consists of Silurian-Devonian siliciclastic flysch, containing olistostromes and olistoliths, overlying a bimodal volcanic-sedimentary complex assigned to the Cambrian-Ordovician (?). Both stratigraphic units underwent post-kinematic contact metamorphism associated with the emplacement of these Pennsylvanian plutons, producing pelitic hornfels, dominated by spotted mica schists with post-kinematic porphyroblasts. Prior to this contact metamorphism, regional M₂ Buchan-type metamorphism produced pre- to syn-kinematic garnet porphyroblasts and syn-kinematic andalusite and staurolite porphyroblasts. These mineral assemblages are associated with the development of a flat-lying pervasive S₂ foliation and mineral lineation, defined by biotite and muscovite (after sillimanite?), which is comparable to that observed in the hanging-wall blocks of Mississippian gneiss domes in the Iberian Massif, including in nearby sectors of the Ossa-Morena Zone. It should also be noted that locally, pre-early-kinematic garnets preserved as cores or as isolated minerals, together with possible high-pressure/low-temperature mineral assemblages in the kyanite zone, were also identified, pointing to a pre-D2 process of regional pressurisation (Barrovian metamorphism), which possibly represents D₁-M₁(?). About 20 km northwest of the Pennsylvanian Figueira e Barros-Ervedal and Fronteira plutons, the Mississippian Ponte-de-Sôr gneiss dome exhibits a pervasive S₂ foliation and top-to-the-SE tectonic transport synchronous with M₂ Buchan-type metamorphism. We propose that a comparable, though cryptic, D₂ gneiss dome developed in the study area prior to the emplacement of the syn- to late-D₃ Figueira e Barros-Ervedal and Fronteira plutons.

Work supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020- https://doi.org/10.54499/LA/P/0068/2020, UID/50019/2025, https://doi.org/10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025, and by the Spanish Ministerio de Ciencia e Innovación, Fondos Feder, PID2023-149105NA-I00. L.S.H. benefits from the FCT PhD scholarship UI/BD/154616/2023, I.D.S from the FCT research contract DL57/2016/CP1479/CT0030 (https://doi.org/10.54499/DL57/2016/CP1479/CT0030), J.C.D. from FCT contract CEECINST/00032/2018/CP1523/CT0002  (https://doi.org/10.54499/CEECINST/00032/2018/CP1523/CT0002), and M.F.P. from grant Nº. FCT/UIDB/06107-Center for Sci-Tech Research in Earth System and Energy-CREATE.