Zeólitas dos Basaltos do Brasil Meridional (Gênese e Pagênese)

Resumo

The basalts and diabases w hich cover an extensive area of the southern States of Brazil are of ten bearers of varied species of zeolites (analcite, chabazite, thomsonite, ptiloli"e, natrolite, scolecite, mesolite, laum ontite, stilbite, stellerite, and heulandite) and as m any other related minerals (delessite, daphnite, celadonite, quartz, chalcedony, calcite, gyroli"e, apophyllite, pyrite, and native copper). In the basalts the above m entioned m inerals occur in cavities of varied forms, w hile in the diabases they are located in the zones of shearing. They originated, mainly, from final residual solutions of the basaltic magm a itself, which were trapped inside the cavities or in the fractures of the diabases. The order of deposi'ion of the m inerals in the am ygdalae and fractures is no" haphazard. On the contrary, there exists a definite sequence, alw ays repeated, in the material examined, no matter what its source. The minerals of the chlorite group and those of the silica group are always the first to be form ed. They are follow ed by the zeolites, then apophyllite, calcite and fin ally the sulphides. Among the zeolites there is also a certain order of deposition. The heulandite precedes the rest. It is follow ed by stilbite, which in i"s turn precedes chabazite. In the zones of shearing of the diabases calcite seems to go contrary to the above order, preceding the deposition of zeolites. There are zones of amygdaloidal basalts in which the cavities are fille d by only one kind of mineral — sometimes only m inerals of the silica group, odier times only analcite, mesolite, or chabazite. Phenomena of pseudomorphic substitutions among the minerals which fill the cavities and fractures of the basalts and diabases are not rare. The inclusions of sandstones and even sands of the Botucatu form ation in the basaltic lava flows did not at all influence the lava which reached the surface. Th ere are no signs of thermal metamorphism between the oldest amygdaloidal basalts and lavas and the most recent diabasic intrusions. The absence of numerous species of zeolites seems to be connected witi the lack of certain elements in the lava and basalts and also the non-existence of differentiated pétrographie types. In the am ygdaloidal basalts, as well as in the homogenous ones and in the other types, as well as in the diabases there are found neither pyrogenic zeolites nor phenomena of analcitization