Petrochronology of the metamorphic belt located west of the Porto-Tomar Shear Zone: variscan geodynamic implications

Abstract

The nature of the metamorphic belt located west of the Porto-Tomar Shear Zone (central-west Portugal) has been abundantly discussed in terms of its pre-Variscan paleogeography (Moreira et al., 2019; Bento dos Santos et al., 2023). However, not enough attention has been given to its metamorphic evolution during the Variscan Orogeny and ensuing deformation/metamorphic stages. In this work, we aim at providing quantitative constraints on the P-T-t paths endured by the high-grade metamorphic rocks for which integrated U-Pb dating of zircon/monazite and Ar-Ar dating of biotite facilitate an innovative petrochronological study revealing its geodynamic evolution. The Porto-Tomar Shear Zone is a first-order structure separating the Iberian Massif domains (to the East) from the often-called Finisterra Terrane (to the west) that is still fully understood. This domain is composed of (at least) 2 metamorphic belts with contrasting metamorphic grades, namely a low-grade and a high-grade metamorphic belt, which some authors correlated, respectively to the South Portuguese Zone (SPZ), and the Ossa-Morena Zone (OMZ; Bento dos Santos et al., 2023), although this interpretation is not unanimous (Moreira et al., 2019). Pseudosection modelling of staurolite-bearing schists, metatexites and diatexites from the high-grade metamorphic belt in the Tomar-Abrantes sector, provides solid P-T constraints to conclude that after a M1 prograde stage at P < 6 kbar, followed a nearly isothermal decompression to P < 3 kbar during the M2 metamorphic peak stage at 680-700ºC, and a M3 stage with isobaric cooling ~500ºC. Additional constraints can be made from U-Pb and Ar-Ar dating of these rocks. U-Pb dating of monazites from one diatexite sample shows 3 specific periods of monazite formation/closure temperature, namely at ~353, ~338 and ~322 Ma. These ages are compatible with what is usually envisaged as the M1 prograde stage, M2 metamorphic peak and M3 cooling path, respectively, for the OMZ. In fact, U-Pb dating of zircon, which typically constrains the metamorphic peak (M2), is conclusive at 343.9 to 338.5 Ma (5 samples). Ar-Ar dating of biotites from 3 samples, which is a good indicator of cooling paths, provides ages between 315.1 to 312.9 Ma, tightly constraining the M3 stage. Integration of the obtained ages with the P-T constraints allow us to define a prograde metamorphic path at ~350 Ma that reached the baric peak at ~6 kbar, whereas the metamorphic peak (T ~ 700º C) was reached at ~342 Ma, after which cooling proceeded. This cooling event is recorded by late formation of monazite at 322 Ma (for a T ~650ºC) and biotite closure temperatures of ~320ºC at ~314 Ma. This P-T-t path is consistent with a 2-stage cooling event: a) ~5ºC/Ma until 322 Ma; followed by b) an increased cooling rate (~27ºC/Ma) until 314 Ma. A D2 extensional event accompanied by asthenospheric upwelling in the OMZ between 340-320 Ma can explain both the slow cooling and the isothermal decompression, whereas a D3 transtensional event from 320 to 310 Ma can explain the increasing cooling rates and the isobaric cooling.