Paraná magmatism and the opening of the South Atlantic

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Abstract
New chemical and isotope results are presented on dyke rocks associated with the Paraná CFB, together with preliminary laser 40 Ar/ 39 Ar analyses on selected Paraná basalts. Dyke rocks from the Ponta Grossa Arch are similar to the Pitanga and Paranapanema magma types in the Paraná lavas, but dykes from the Santos-Rio de Janeiro section include samples with compositions not observed in the overlying lavas. Rather their minor and trace elements are strikingly similar to basalts recently erupted on Tristan de Cunha, and thus these late stage dykes may represent the first direct evidence for the involvement of typical plume-related OIB in the Paraná province. Laser 40 Ar/ 39 Ar analyses of two Gramado low Ti basalts have yielded preferred isochron ages of 132.4 ± 1.4 and 132.9 ± 2.8 Ma. These indicate a short eruption time for at least the Gramado magma type, and that magmatism took place several million years after the species extinction in the Tithonian ( c. 141 Ma). The majority of basalts and basaltic andesites in the Paraná CFB have distinctive trace elements ratios (low Nb/La and Nb/Ba), and relatively enriched Sr, Nd, and Pb isotope compositions. Since such features are not commonly observed in oceanic basalts, and they occur in CFBs which have been screened for the effects of crustal contamination, they are typically attributed to old, incompatible element enriched source regions in the continental mantle lithosphere. In some models the minor and trace element ‘mantle lithosphere’ component was introduced in small degree melts (lamproites) added to asthenosphere derived magmas. However, such models appear to be inconsistent with the data from low Ti CFB, and they also require that the asthenosphere derived magmas have very low incompatible element contents, in marked contrast to the high Nb/La late stage dykes in the Paraná. Alternatively some CFBs may have been generated within the mantle lithosphere in the presence of small amounts of water. The results of preliminary calculations indicate that in the presence of a mantle plume up to 5 km of melt may be generated entirely from within the mechanical boundary layer, for β values of less than 1.2.