PETROGRAFIA E GEOQUÍMICA DOS DIQUERS FÉLSICOS E MÁFICOS ASSOCIADOS AO BATÓLITO PELOTAS NA REGIÃO DE PIRATINI, RS

Abstract

Petrographic studies on the mafic and felsic dykes from the Piratini region, RS, reveal a porphyritic texture with an aphanitic to fine-grained matriz, with glomeroporphyritic and spherulitic textures in the case of felsic, and ophitic to sub-ophitic with myrmekitic intergrowths in the case of mafic dykes. These dykes are intrusive in granitic rocks of the Pelotas Batholith along NW-SE and N-S trends associated with high angle shear zones. They are strongly metaluminous (mafic) and slighty metaluminous to peraluminous (felsic), with an alumina saturation index between 0.60 and 0,65 and 0,75 and 1,2, respectively. The Si02 content in the mafic dykes varies from 44 to 48 wt % and in the felsic varies from 67 to 75 wt %. The mafic dykes present higher Ti, Mg, Ca, Fe, Mn and P contents in comparison with the felsic dykes. The behavior of several major and trace elements (Fe, Mn, Mg, Ti, P, Ca and Sr, and, in minor degree, Ca and Mg show the importance of crystallization of iron-magnesium and feldspars in both dykes. The Nb/Ta e U/Th ratios of trace elements of these rocks show evidence of crustal contamination. The HREE display sub horizontallized patterns, showing Tb/Lu ratios from 1,5 to 2,7 in the samples of mafic dykes and from 1,6 to 2,5 in the felsic dykes. The samples have shown Lu concentration from 0,3 ppm to 0,6 ppm in mafic, and from 18 ppm to 41 ppm in the felsic dykes, respectively. The felsic dykes are more enriched in LREE, with La values from 73 ppm to 155 ppm, whereas mafic dykes present 18 ppm to 41 ppm of La and La/Sm ratios in the range of 3,0 to 5,0.