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

The petrology and palaeomagnetism of basic and acidic volcanic rocks of the Paranà plateau (South Brazil) have been investigated. The lower sections of the sequence are largely composed of tholeiitic basalts and tholeiitic basaltic andesites while the upper portions are essentially represented by rhyodacites showing intercalations of basaltic and/or andesitic rock-types. Tholeiitic andesites prevail at the basic-acidic boundary. Petrography, mineral chemistry and bulk-rock composition (major and trace elements) data clearly define tholeiitic suites displaying possible liquid lines of descent related to different degrees of partial melting and crystal fractionation controls. Mass balance calculations, based on phenocrysts and/or microphenocrysts present in the different rock-types are consistent with the hypothesis that rhyodacitic melts may be derived from basalts through low-pressure crystal fractionation processes accompanied by crustal contamination. Palaeomagnetic data indicate high rates of magma emission (e.g. 700 m in <1 m.y.) and sometimes the contemporaneous eruption in different areas of basic and acidic lavas. Moreover, the positions of the palaeomagnetic poles indicate that the volcanic fields of Paranà and Namibia (southern Africa) were joined at 120 m.y. B.P. All the data consistently indicate that Paranà volcanism occurred during the Lower Cretaceous after the formation of up-doming structures, and that the initial opening of the South Atlantic Ocean, at the latitudes of the Paranà basin, occurred not before 120 m.y. B.P.