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The link between volcanism and the Aljustrel VHMS deposit : geochemistry, geochronology and insights on the paleogeography, provenance and geotectonic setting of the Iberian Pyrite Belt
Publication . Amaral, João; Cruz, Ana Rita Zeferino Solá da; Santos, Telmo Manuel Bento dos
This thesis delves into the petrogenesis of the bimodal volcanism of the Aljustrel polymetallic deposit, located in the Iberian Pyrite Belt (IPB), South Portuguese Zone (SPZ), Iberian Massif, as well as the relation of magmatic events with the formation of VMS mineral deposits. New Zircon SHRIMP and LA-ICP-MS U-Pb and whole-rock Sm-Nd isotopic, and major and trace element analysis are presented to characterize the Aljustrel’s volcanism and contribute to the understanding of the geodynamic evolution of the IPB. In Aljustrel, felsic volcanism took place between 359 and 353 Ma. The maximum ages obtained for each deposit opens the possibility that the last pulses of volcanic activity and subsequent massive sulphides deposition were diachronous in Aljustrel’s sub-basins. Devonian inherited ages presented in both isotopically juvenile (ƐNdi = +1.79) and more evolved (ƐNdi = -5.07) felsic rocks implies a heterogeneous igneous zircon-rich source and a long-lived magmatic activity (~50-70 Ma). Ga/Al and Y/Nb of the felsic volcanism points to A2-type affinities. Coeval mafic rocks show orogenic signatures attributed to a mantle modified by subduction metasomatism (ƐNdi = +1.54 to +5.48). The geochemical and isotopic signatures of Aljustrel’s bimodal volcanism suggest a post-collisional or back-arc setting for the Iberian VMS deposits. A geodynamic model that includes a continuous evolution from Devonian to Carboniferous magmatism with subduction cessation and asthenospheric rise after the Meguma-Avalonia neo Acadian collision, followed by an intracontinental extensional setting as a result of Gondwana - Meguma Variscan collision is proposed. It is also considered that the IPB diachronic volcanism might have resulted from Rheic’s slab roll-back and tearing. Considering SPZ’s zircon age-distribution three detrital groups were identified: i) a group showing similarities with the West African Craton; ii) a group containing a significant Mesoproterozoic population; and iii) a group with significant Carboniferous ages. A provenance from Meguma for the late Devonian IPB detrital rocks is demonstrated and the remarkable similarities between Ossa-Morena and Meguma detrital rocks suggest a similar paleogeographic setting relatively to the North African blocks on Ediacaran-Ordovician times. Taken together, the 1.8-2.4 Ga age-distribution is a valuable paleogeographic-provenance proxy.
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Fundação para a Ciência e a Tecnologia
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SFRH/BD/138791/2018