Flood Basalt to Rhyolite Suites in the Southern Paran? Plateau (Brazil): Palaeomagnetism, Petrogenesis and Geodynamic Implications

The petrology and palaeomagnetism of basic and acidic volcanicrocks of the Paran? plateau (South Brazil) have been investigated.The lower sections of the sequence are largely composed of tholeiiticbasalts and tholeiitic basaltic andesites while the upper portionsare essentially represented by rhyodacites showing intercalationsof basaltic and/or andesitic rock-types. Tholeiitic andesitesprevail at the basic-acidic boundary. Petrography, mineral chemistry and bulk-rock composition (majorand trace elements) data clearly define tholeiitic suites displayingpossible liquid lines of descent related to different degreesof partial melting and crystal fractionation controls. Mass balance calculations, based on phenocrysts and/or microphenocrystspresent in the different rock-types are consistent with thehypothesis that rhyodacitic melts may be derived from basaltsthrough low-pressure crystal fractionation processes accompaniedby crustal contamination. Palaeomagnetic data indicate high rates of magma emission (e.g.700 m in <1 m.y.) and sometimes the contemporaneous eruptionin different areas of basic and acidic lavas. Moreover, thepositions of the palaeomagnetic poles indicate that the volcanicfields of Paran? and Namibia (southern Africa) were joined at120 m.y. B.P. All the data consistently indicate that Paran? volcanism occurredduring the Lower Cretaceous after the formation of up-domingstructures, and that the initial opening of the South AtlanticOcean, at the latitudes of the Paran? basin, occurred not before120 m.y. B.P. * Reprint requests to E. M. Piccirillo.