Geochronological constraints on the late to post-Variscan hypabyssal dykes from northern Portugal and their geodynamic implications

The Permo-Carboniferous magmatism, recorded throughout NW and SW Europe, is related to the late to post-tectonic stages of the Variscan orogeny and constitutes an important period of reorganization of the stress field due to the transition from a compressive/transpressive to extensional/transtensive setting. In northern Portugal, this event manifested through the presence of numerous dykes, sills, and masses of subvolcanic lithologies such as granite porphyries, lamprophyres, and dolerites. The present study is focused on the geochronological results acquired from zircon U-Pb dating of selected dykes, namely the Póvoa de Agrações (PA) and Vila Nova de Foz Côa (VNFC) granite porphyries, the Lamas de Olo (LO) lamprophyre, and the Bolideira quartzdiorite porphyry, and the interpretation of these results from a geodynamic perspective.

Attending only to the most concordant analytical spots (discordance < 5 to 10%) with appropriate ages, corrected for common Pb, weighted means of the ²⁰⁶Pb/²³⁸U age yield the following crystallization values: (i) 286 ± 1.5 Ma (MSWD = 0.3) (PA); (ii) 290 ± 6 Ma (MSWD = 2.5) (VNFC); (iii) 295 ± 2 Ma (MSWD = 2.5) (LO); and (iv) 291 ± 5 Ma (MSWD = 3.8) (Bolideira). The porphyries also exhibit two sets of inherited zircon cores, an older one (broadly Paleoproterozoic to Mesoproterozoic) and a younger counterpart (Neoproterozoic (Cryogenian) to Early Silurian), while in the LO lamprophyre, the composing inherited cores are Neoproterozoic (Ediacaran) to Early Ordovician (Floian). For the PA and VNFC dykes, the existence of two inherited core components is possibly associated with the distinct protolith contributions (i.e., metapelites and metagreywacke/orthogneiss) involved in the petrogenesis of these lithotypes, as deduced from the whole-rock geochemistry, whereas the presence of inherited cores in the Bolideira porphyry is most likely related to crustal contamination. Moreover, the inherited cores of the LO lamprophyre are interpreted to have resulted from sediment-induced, metasomatic enrichment of its lithospheric mantle source during subduction.

Based on the aforementioned geochronological constraints, the acid, intermediate and mafic subvolcanic rocks analyzed within the scope of this work are, in fact, late to post-Variscan and, most importantly, (sub)contemporaneous. The prior observation is possibly valid for several other hypabyssal dykes in northern Portugal that have yet to be dated, considering their general orientations and similarities concerning the bulk-rock composition. Therefore, assuming that the regional felsic, intermediate, and mafic (lithosphere-derived and asthenosphere-derived) subvolcanic specimens are roughly coetaneous, the geodynamic evolution of the Central Iberian Zone during the post-Variscan stages is implied to have progressed more rapidly than previously thought, due to the presumed coeval emplacement of distinct mafic melts generated from both lithospheric and asthenospheric sources.

This work was supported by national funding awarded by FCT – Foundation for Science and Technology, I.P., projects UIDB/04683/2020 and UIDP/04683/2020. The main author is financially supported by FCT through an individual Ph.D. grant (reference SFRH/BD/138818/2018). We also acknowledge Professor Pilar Montero (IBERSIMS, University of Granada) for performing the geochronological analyses.