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- The bimodal Fii-A2-type and calc-alkaline volcanic sequence of the Aljustrel brownfield region, Iberian Pyrite Belt, SW Iberian MassifPublication . Lains Amaral, João; Solá, A. Rita; Santos, Telmo M. Bento Dos; Feitoza, L. M.; Tassinari, Colombo C. G.; Crispim, Lourenço; Chichorro, Martim; Hofmann, Mandy; Gãertner, Jessica; Linnemann, Ulf; Gonçalves, JoãoABSTRACT: The Iberian Pyrite Belt (IPB) is a late Devonian- Early Carboniferous world-class polymetallic VMS province that includes significant Cu-(Sn)-Pb-Zn-(Ag) deposits of massive sulphides and feeder zones. The Aljustrel brownfield region contains one of the highest concentrations of ore in the IPB in 6 known deposits (Gaviao, Sao Joao, Moinho, Algares, Estacao and Feitais). To delve into the petrogenesis of the Aljustrel early Carboniferous (similar to 355 Ma) felsic-dominated bimodal volcanism, new whole-rock trace elements and Sm-Nd isotopes, and U-Pb in zircon were obtained. Based on Ga/Al and Y/Nb ratios, it is shown that Aljustrel felsic magmatism has the geochemical features of A2-type melts, typical of post-collisional and back-arc settings. U-Pb in zircon for a juvenile felsic volcanic rock point to antecrysts ages spanning from 387.9 to 366.6 Ma and a maximum emplacement age of 354.3 +/- 2.6 Ma. These long-lasting melting events, present in both juvenile (epsilon Ndi = +1.79) and evolved felsic rocks (epsilon Ndi =-5.07), imply heterogeneous sources dominated by zircon-bearing igneous rocks. The Sm-Nd model ages are in accordance with previous Lu-Hf model ages in zircon, reinforcing that the isotopic variability is related to the same petrogenetic process. Subordinated Aljustrel mafic rocks, coeval with the abundant felsic volcanism, show orogenic signatures, namely Nb-Ta-Ti negative anomalies and calc-alkaline affinities, whereas Sm-Nd isotopic data (epsilon Ndi = +1.54 to +5.48) points to variable to no contamination with crustal material. These geochemical results suggest derivation from an enriched mantle source modified by subduction metasomatism. In addition, the mafic rocks did not provide zircons for geochronological analysis, with the exception of one sample, in which a Concordia age of 402.1 +/- 15.5 Ma was obtained from a single grain. The combined geochemical signatures of mafic and felsic volcanic rocks suggest asthenospheric rise, but this solely does not explain the abundance of zircon antecrysts in the felsic rocks. Therefore, a geodynamic model that includes a continuous evolution from Devonian to Carboniferous times is inferred. This more complex and broader geodynamic model for the Iberian Pyrite Belt in which successive metal remobilization occurred after successive melting events, fits the present geochemical data and is more likely to explain why the Iberian Pyrite Belt is a unique metallogenetic province.
- Novos dados U-Pb em zircão do Maciço de Évora e correlação com as unidades basais da Zona de Galiza-Trás-os-MontesPublication . Solá, A. Rita; Chichorro, Martim; Cachapuz, P.; Santos, Telmo M. Bento DosSUMMARY: We present new U-Pb zircon data from the Évora Massif (SW Ossa–Morena Zone). Série Negra paragneiss of Escoural Fm. yields a maximum depositional age of ~580 Ma. Felsic orthogneiss “leptinites” included in Escoural Fm., commonly attributed to the Precambrian, yield a concordant age of 513 ± 3 Ma. Felsic volcanics interbedded in the Monfurado Fm. and the Safira Orthogneiss show very discordant zircon ages and two systematic populations at ~510 Ma and ~460 Ma. Given the stratigraphic constraint of Monfurado Fm., both magmatic rocks are considered coeval and emplaced at ~510 Ma during the main rift-related magmatism. The ~460 Ma age is not fully understood yet. Given the differences in the ages of magmatism in the Évora Massif, recently correlated with the Lower Allochthon of Galiza-Trás-os-Montes Zone, where only late rift-related magmatism is recorded, we suggest that they represent different sedimentary basins positioned in the same paleogeographic contexts.