Actas de congresos
Age of the Paraná continental flood basalts using 187Re·1870s systematics
Fecha
2014Registro en:
South American Symposium on Isotope Geology, 9, 2014, São Paulo
Autor
Júnior, Eduardo Reis Viana Rocha
Marques, Leila Soares
Babinski, Marly
Petronilho, Liliane Aparecida
Machado, Fábio Braz
Nardy, Antonio José Ranalli
Institución
Resumen
A suite of basaltic rocks sampled over a vast exposure in the Paraná continental f100d basalts (PCFB; [1]) has been investigated for Os isotopic systematics, also including olivine phenocrysts of Spitz- koppe picrites of the Etendeka[2]. Rhenium and osmium concentrations for basalts vary between 0.55 and 1.1 ng/g and 0.033 and 0.13 ng/g, respectively. The 187Re/1880s ratios for basalts range from -30 to 90. The olivine phenocrysts have: [Re] = 0.18 and 0.27 ng/g; [Os] = 1.05 and 0.84 ng/g; 187Re/1880s
= 0.81 and 1.55. On a plot of 187Re/1880s versus 1870S/1880S, the Re-Os data for 13 samples of both high- and low- Ti basalts, and olivine phenocrysts form a linear trend with a slope Ihat corresponds to an age of 134.2 ± 2.4 Ma (MSWD = 19) and an initial1870s/188Os = 0.1271 ± 0.0019. The 187Re-1870s age is in agreement with previous results based on the 40Ar/39Ar plateau (134-132 Ma) method and U-Pb (134.3 Ma) dates. As we obtain a well defined isochron for the PCFB' basalts and Etendeka' olivine phenocrysts, implies that the source of the magmatism was isotopically homogeneous over the geographic extent and the duration of the volcanism. Also, if we add to our model one basalt (average of 7 duplicates) of the PCFB, with 187Re/1880s<100 (uncertainty in initial 1870s/188Os is<3%). published
by [3], we obtain a Re-Os age of 134.0 ± 2.4 Ma (MSWD = 19) and an initiaI 187Os/188Os=O.1270 = 0.0019. These initial 1870s/1880s overlap with the range of compositions observed for the convecting upper mantle. It also overlaps with the compositions of fertile SCLM xenoliths from the spatiaIy associated Goiás Alkaline Province. Additional analysis of the Ribeira magma-type (in progress). wtlich was
not already investigated for Re-Os, will shed further light on the generation of lhe PCFB. particúarty its northwestern portion. [1] Rocha-Júnior et al. (2012) Earth Planet. Sci Lett. 337-338: 164-173 [2] Thompson et al. (2007) J. Petrol. 48: 1119-1154 [3] Correia et al. (2011) Cong. Bras. Geoquímica 1143-1146