Paraná magmatism and the opening of the South Atlantic

C. J Hawkesworth; K Gallagher; S Kelley; M Mantovani; D. W Peate; M Regelous; N. W Rogers

doi:10.1144/GSL.SP.1992.068.01.14

New chemical and isotope results are presented on dyke rocks associated with the Paran~ CFB, together with preliminary laser 'l°Ar/39Ar analyses on selected Paran,'i basalts. Dyke rocks from the Ponta Grossa Arch are similar to the Pitanga and Paranapanema magma types in the Paran~i lavas, but dykes from the Santos--Rio de Janeiro section include samples with compositions not observed in the overlying lava s. 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 Paran~i province. Laser 4°Ar/39Ar analyses of two Gramado low Ti basalts have yielded preferred isoehron 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 Paran~i 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 Paran~i. 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/3 values of less than 1.2.

Paraná magmatism and the opening of the South Atlantic

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