Age of the Ponta Grossa dike swarm (Brazil), and implications to Paraná flood volcanism

Details Age of the Ponta Grossa dike swarm (Brazil), and implications to Paraná flood volcanism Age of the Ponta Grossa dike swarm (Brazil), and implications to Paraná flood volcanism

Oct 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996e%26psl.144..199r&link_type=abstract

Earth and Planetary Science Letters, Volume 144, Issue 1-2, p. 199-211.

Mathematics

Logic

33

Scientific paper

The Ponta Grossa Dike Swarm (PGDS) occurs in a NW-trending, 200 km wide zone exposed just east of the Paraná basin in southeastern Brazil. The predominantly basaltic dikes intrude crystalline basement, Paleozoic-Mesozoic sediments, and (rarely) flows of the Paraná-Etendeka flood volcanic province (PEP). The PGDS resembles the failed arm of a rift-rift-rift triple junction, related to the separation of South America and Africa. Detailed geochemical studies of the dikes (including major/minor/trace element and Sr-Nd isotopic analyses) indicate that they probably represent feeders for the voluminous phase of flood volcanism, represented by relatively uncontaminated, predominantly high-TiO2 lavas of the northern PEP, where lava accumulations reach 1700 m thick. 40Ar/39Ar stepwise degassing data, using both laser and radiofrequency induction furnace, on plagioclase separates from eighteen dikes and one 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 from the plateau ages, and plateau ages are adopted in all but two cases as being the most accurate age estimates. The age-probability distribution for the dominant pulse (131.4 +/- 0.4 to 129.2 +/- 0.4 Ma) shows a pronounced peak at 130.5 Ma; this distribution probably reflects the magma production history in the region. The new geochronologic data are consistent with conclusions based on paleomagnetic and chemical-stratigraphic data that the PGDS is younger than the volumetrically dominant majority of volcanism in the southern PEP, which occurred at 133-132 Ma. The younger (commonly NE-trending) dikes may reflect the initiation of full drift, which was coincident with major basin development in the 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 site that would be inferred for plume impact.

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