MAGMATISM AND CONTINENTAL BREAK-UP IN THE SOUTH-ATLANTIC - HIGH-PRECISION AR-40-AR-39 GEOCHRONOLOGY

A detailed Ar-Ar study of the Parana-Etendeka continental flood basalt s (CFB) has been undertaken using laser spot analysis. Data provide in formation not only on the age of the samples but also on the variabili ty of the non-radiogenic Ar component and state of alteration. The res ults indicate the Parana-Etendeka CFB were erupted over 10 million yea rs between 137 and 127 Ma providing a minimum overall eruption rate of approximately 0.1 km3 yr-1. This is an order of magnitude less than t hat previously proposed for this, the Deccan and Siberian CFB province s but it is similar to estimates for Hawaii, Iceland and the Columbia River CFB. In detail, the new Ar-Ar analyses indicate that magmatism w ithin the Parana-Etendeka province migrated from NW to SE prior to and during the opening of the South Atlantic, providing an explanation fo r the asymmetry of the CFB lavas about the South Atlantic. Moreover, c hemically defined magma types were erupted at different times in diffe rent places, and so within the Parana they may not be used as reliable chronostratigraphic units. Rather, such magmatic units may reflect th e extent of compositionally distinct source regions in the uppermost m antle, and indicate that partial melting took place over a wide area u nder the Parana Basin. The NW-SE migration of the onset of magmatism m ight be interpreted as a plume trace, but the inferred rate of movemen t is 3 times faster than that inferred from subsequent magmatism on th e Rio Grande Rise. It is argued that the onset of magmatism reflected extensional tectonics normal to the Ponta Grossa dyke swarm. Given tha t magmatism preceded the main phase of rifting, and that extension acr oss the Ponta Grossa was moderate, current plume models have difficult y in predicting sufficient melting within the asthenosphere.