Quimismo dos minerais máficos e óxidos em diques alcalinos e de composições básicas a intermediárias da região costeira entre São Sebastião e Ubatuba, estado de São Paulo

Abstract

Along the northern coast of São Paulo State (Brazil), alkaline and basic to intermediate dykes strike parallel to NE-SW-trending lineaments in the Precambrian Costeiro Complex. Alkaline compositions range from picritic lamprophyres, camptonites, monchiquites, biotite lamprophyres, tephrites and rarer alnöites, whereas the basic to intermediate are classified chemically as basalts, trachybasalts and basaltic trachyandesites. The alkaline dykes contain forsteritic olivines with 72 < Fo < 88, except for those in the tephrites, where 46 < Fo < 68. Alnöites and biotite lamprophyres present zoned biotites characterized by Fe and Al increase, erratic Ti behaviour and Mg decrease from (phlogopitic) core to (biotitic) rim. For the biotites from other lamprophyres and tephrites, Ti decreases while Fe increases, whereas for those in basic to intermediate dykes Ti and Fe increase concomitantly. The alkaline dykes present zoned chrome spinels where Al, Mg and Cr decrease while Ti and Fe increase from core to rim. Al and Mg contents are highest in alnöite Fe-Ti spinels. In the other lamprophyres, Al and Mg decrease while Fe increases in Fe-Ti spinels. Relatively lower Al contents and lack of Mg characterize Fe-Ti spinels (titanomagnetites) from basalts, trachybasalts and basaltic trachyandesites. Ilmenites appear only in these rocks. Mg and Al are characteristic elements of mafic and oxide minerals whereas Fe and Ti characterize these minerals in basic to intermediate dykes. Alnöites and biotite lamprophyres stand out due to their carbonatitic affinity, reflected, for instance, in their pyroxene compositions. The other lamprophyres and tephrites must have evolved from alkaline basalts, whereas dykes of basic to intermediate compositions show affinities with basaltic rocks of the Paraná Basin.