Basalt magmatism along the passive continental margin of SE Brazil (Campos basin) |
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| Authors: | A M P Mizusaki R Petrini P Bellieni P Comin-Chiaramonti J Dias A De Min E M Piccirillo |
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| Institution: | (1) Petrobras (Cenpes/Divex/Setec), Cidade Universitaria, Quadra 7, Ilha do Fundao, 21910 Rio de Janeiro, RJ, Brazil;(2) Laboratorio di Geocronologia e Geochimica Isotopica, CNR, Pisa, Italy;(3) Dipartimento di Mineralogia e Petrologia, University of Padova, Italy;(4) Istituto di Mineralogia, Petrografia e Geochimica, University of Palermo, Italy;(5) Petrobras (Depex/Secamp), Rio de Janeiro, RJ, Brazil;(6) Istituto di Mineralogia e Petrografia, University of Trieste, Piazzale Europa, 1, I-34100 Trieste, Italy |
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| Abstract: | The SE-Brazil passive continental margin is characterized by tholeiitic magmatism that is particularly widespread in the marginal Campos basin, facing the inland flood basalts of the adjacent Paranà basin. Campos magmatism is represented by Early Cretaceous (EC; 134–122 Ma) flood basalts and minor Upper Cretaceous-Early Tertiary basalt flows and intrusives, which were emplaced in a basin with attenuated crustal thickness (20 km). Petrography, mineral chemistry, wholerock geochemistry and Sr–Nd isotope composition emphasize that the EC-Campos basalts have suffered extensive seawater interaction which caused enrichment in MgO, FeO total, K2O, Rb and Ba, and depletion in SiO2 and CaO, while Zr, Nb, Y and REE remained virtually constant in samples with loss-on-ignition values less than 4 wt%. In general, Campos basalts have bulkrock chemistry similar to those of the inland Parana tholeiites (140–130 Ma) with relatively low concentrations of incompatible elements and TiO2 (<2 wt%). Batch-melting calculations suggest that Campos basalt genesis requires a garnet-peridotite source and variable degree of melting (9–25%) in order to explain the rare-earth-element (REE) patterns with chondrite normalised La/Yb(N) ratio ranging from 0.9–1.0 to 4.4–7.1. The Sr–Nd isotopic data for the slightly altered Campos basalts, of both Early Cretaceous and Upper Cretaceous-Early Tertiary age, plot close to bulk earth, or in the enriched quadrant of the mantle array. Inter-element (La, Zr, Nb, Ba) ratios preclude for the Campos (and Paranà) basalt genesis any significant participation of N-type (Zr/Nb>16) MORB (mid-ocean-ridge basalt) mantle in simple binary mixing models. On the whole, the Early Cretaceous Campos basalts appear as an easterly, younger extension of the northern Paranà volcanism and probably erupted during early stages of the major riftingprocesses which caused continental thinning. It is notable that in the Campos marginal basin both the basalt magmatism contemporaneous with the continental break-up, as well as that which occurred after the S. America-Africa separation, appears substantially related to subcontinental lithosphere and a Dupal-like OIB (ocean-island basalt) (e.g. Tristan da Cunha) source components. |
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