Potassium-argon dates of basaltic rocks from Southern Brazil

Abstract

Potassium-argon ages are reported for 20 basalts (including a few core samples) and 15 diabases from the Paraná basin of Southern Brazil. Histograms of both basalt and diabase ages exhibit strong peaks close to 120 m.y. Thus the principal Brazilian volcanism was Mid-Lower Cretaceous and not Jurassic as earlier supposed. While the few dates younger than 120 m.y. are likely due to argon loss, there is a real “tail” of higher ages which shows that there was precursory volcanic activity 147 m.y. ago, in the Upper Jurassic, and perhaps earlier. The span of at least 28 m.y. for the volcanism is similar to that which has been found for South Africa. The basaltic rocks of Brazil differ, however, both in age and potassium content from the older diabase rocks of Southeast Africa, Antarctica and Tasmania which have been dated thus far. Auxiliary studies included a careful evaluation of the analytical precision in our techniques and control experiments in dating series of spaced samples from narrow diabase dikes which represent examples of “instantaneous” cooling. The latter experiments show that whole-rock dating of diabase from Brazil is occasionally susceptible to discrepancies of up to 8 per cent even when fairly rigid thin-section criteria have been exercised in the selection of the samples for dating. An argon diffusion experiment on diabase material gave results characteristic of feldspar. Acid etching was found not to be effective in “rectifying” discordant feldspar separates. Auxiliary studies by electron microprobe showed that a diabase sample, in which only plagioclase feldspar would be noted in cursory visual examination, actually contains feldspars ranging from plagioclase through potash oligoclase and anorthoclase to sodic sanidine, where inconspicuous weathering of the last might be important in creating discrepancies of the type noted.