Magmatism and continental break-up in the South Atlantic: high precision 40Ar39Ar geochronology

A detailed ArAr study of the Paraná-Etendeka continental flood basalts (CFB) has been undertaken using laser spot analysis. Data provide information not only on the age of the samples but also on the variability of the non-radiogenic Ar component and state of alteration. The results indicate the Paraná-Etendeka CFB were erupted over 10 million years between 137 and 127 Ma providing a minimum overall eruption rate of ∼ 0.1 km³ yr^-1. This is an order of magnitude less than that previously proposed for this, the Deccan and Siberian CFB provinces but it is similar to estimates for Hawaii, Iceland and the Columbia River CFB. In detail, the new ArAr analyses indicate that magmatism within the Paraná-Etendeka province migrated from NW to SE prior to and during the opening of the South Atlantic, providing an explanation for the asymmetry of the CFB lavas about the South Atlantic. Moreover, chemically defined magma types were erupted at different times in different places, and so within the Paraná they may not be used as reliable chronostratigraphic units. Rather, such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle, and indicate that partial melting took place over a wide area under the Paraná Basin. The NW-SE migration of the onset of magmatism might be interpreted as a plume trace, but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the Rio Grande Rise. It is argued that the onset of magmatism reflected extensional tectonics normal to the Ponta Grossa dyke swarm. Given that magmatism preceded the main phase of rifting, and that extension across the Ponta Grossa was moderate, current plume models have difficulty in predicting sufficient melting within the asthenosphere.