AGE OF THE PONTA-GROSSA DIKE SWARM (BRAZIL), AND IMPLICATIONS TO PARANA FLOOD VOLCANISM

The Ponta Grossa Dike Swarm (PGDS) occurs in a NW-trending, 200 km wid e zone exposed just east of the Parana basin in southeastern Brazil. T he predominantly basaltic dikes intrude crystalline basement, Paleozoi c-Mesozoic sediments, and (rarely) flows of the Parana-Etendeka flood volcanic province (PEP). The PGDS resembles the failed arm of a rift-r ift-rift triple junction, related to the separation of South America a nd Africa. Detailed geochemical studies of the dikes (including major/ minor/trace element and Sr-Nd isotopic analyses) indicate that they pr obably represent feeders for the voluminous phase of flood volcanism, represented by relatively uncontaminated, predominantly high-TiO2 lava s of the northern PEP, where lava accumulations reach 1700 m thick. Ar -40/Ar-39 stepwise degassing data, using both laser and radiofrequency induction furnace, on plagioclase separates from eighteen dikes and o ne sill yield seventeen plateau ages: three are between 120.7+/-1.3 Ma and 125.8+/-0.6 Ma, and fourteen are clustered between 129.2+/-0.4 Ma and 131.4+/-0.5 Ma. Isochron ages are not significantly different fro m the plateau ages, and plateau ages are adopted in all but two cases as being the most accurate age estimates. The age-probability distribu tion for the dominant pulse (131.3+/-0.4 to 129.2+/-0.4 Ma) shows a pr onounced peak at 130.5 Ma; this distribution probably reflects the mag ma production history in the region. The new geochronologic data are c onsistent with conclusions based on paleomagnetic and chemical-stratig raphic data that the PGDS is younger than the volumetrically dominant majority of volcanism in the southern PEP, which occurred at 133-132 M a. The younger (commonly NE-trending) dikes may reflect the initiation of full drift, which was coincident with major basin development in t he adjacent continental borderland at 125-120 Ma. The PGDS may indeed represent the failed arm of a rift-rift-rift triple junction, but the triple junction did not coincide exactly in time or space with the sit e that would be inferred for plume impact.