Variscan plutonism in the Ossa-Morena Zone (SW Iberian Massif): The Mississippian and Pennsylvanian magmatism and its importance for the regional tectonic sequence of events

The Variscan orogen marks the late Devonian-Carboniferous collision of Gondwana and Laurrusia, forming Pangea.  Throughout this orogen, extensive granitic plutons mark the tectonic and thermal processes behind its origin and development. Their study provides a unique opportunity to reconstruct the tectonic sequences, interpret the stages of crustal evolution, and assess the overall mechanics of complex lithospheric processes.

The Iberian Massif, located at Pangaea’s core, has one of the best exposures of the Variscan orogen in Europe, facilitating the study of deep-to-surface geodynamic phenomena. Ongoing research in the SW Iberian Massif (Ossa-Morena Zone), reveals a close relationship between the deformation, metamorphism, magmatism and sedimentary processes involved in deep to shallow lithospheric dynamics, during both orogenic thickening and gravitational collapse. Multiple magmatic stages mark these events, whose records include multiple granitic plutons (e.g.,[1], [2], [3]).

New data, obtained through the application of modern geological mapping techniques (geochemistry, geochronology and microtectonics) to the Figueira e Barros/Ervedal  (307 Ma), and Fronteira (308 Ma) plutons indicate that these are calc-alkaline, aluminous, syn- to post-kinematic granitic intrusions, that cross-cut the Mississippian and Pennsylvanian heterogeneous fabrics formed under low metamorphic conditions (D2-M2 and D3-M3 stages) [4], probably formed in a volcanic arc environment. The surrounding lithologies are mostly composed of Devonian-Silurian(?) schists and slates, with olistoliths and volcanic rocks, probably part of a flysch synorogenic sequence. These units were affected by (i) contact metamorphism in the vicinity of the plutons, locally characterised by spotted mica-schists and cut by late Variscan-early Alpine(?) NW-SE/N-S faults; (ii) a Buchan type HT-LP metamorphism associated to the Ponte de Sor gneiss dome that enabled the blastesis of Mississippian syn-D2-M2 garnet, andalusite and staurolite.

This new data allows for a better definition of the regional sequence of events, and for a comparison between the studied plutons and the ones found in the bibliography (e.g., Benavila [1], Stª Eulália [2], Pavia [3]) which in turn constrains the regional conceptual model for the Variscan Orogeny.

This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020), 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) and J.C.D. from FCT research contract CEECINST/00032/2018/CP1523/CT0002  (https://doi.org/10.54499/CEECINST/00032/2018/CP1523/CT0002).

 

[1] Canilho, M.H., 1992. Contribuição para o conhecimento petrográfico e geoquímico do maciço ígneo de Benavila (Avis). Ciências da Terra, 11, pp.1004–1018.

[2] Pereira, M.F., et al., C., 2017. Coeval interaction between magmas of contrasting composition (Late Carboniferous-Early Permian Santa Eulália-Monforte massif, Ossa-Morena Zone): field relationships and geochronological constraints. Geologica Acta, 15(4), pp.409–428. 10.1344/GeologicaActa2017.15.4.10

[3] Lima, S.M., et al., 2012. Dissecting complex magmatic processes: An in-depth U–Pb study of the Pavia pluton, Ossa–Morena Zone, Portugal. Journal of Petrology, 53(9), pp.1887–1911. https://doi.org/10.1093/petrology/egs037

[4] Dias da Silva, Í., et al., 2018. Time-space distribution of silicic plutonism in a gneiss dome of the Iberian Variscan Belt: The Évora Massif (Ossa-Morena Zone, Portugal). Tectonophysics, 747-748, 298-317. https://doi.org/10.1016/j.tecto.2018.10.015