Propriedades Magnéticas das Rochas Vulcânicas Mesozóicas da Bacia do Maranhão

Abstract

Two paleomagnetic poles were determined from. respectively, 'l 13 samples of Jurassic volcanic rocks, lyinq in the western part of Maranhão Basin (Porto Franco-Estreito) and 157 samples of sills and dikes of lower Cretaceous age (Teresina-Picos-Floriano) from the eastern part. The rock magnetic studies of these samples are presented in this paper. Analysis of the magnetic minerals of the samples by thermomagnetic curves and X-ray diffraction yielded, in most cases titanomagnetites with poor titanium content as Weilas a phase near ilmenite. This can be interpreted as high temperature oxidation during rock formation. Generally determined Curie. temperatures lay between 500-6000C. All titanomagnetites are more or less low temperature oxidized (maghemized). The oxidation is more advanced in the samples from the western part of the Maranhão basin than from the eastern part. The composition of the titanomagnetites is distributed along an "oxidation line" in the ternary system with a trend showing loss of iron with increasing oxidation.