Caracterização geoquímica preliminar dos granttóides aflorantes nas vizinhanças do batólito Pinhal-Ipuiúna (SP-MG)

Abstract

PRELIMINARY GEOCHEMICAL CHARACTERIZATION OF GRANITOIDS OUTCROPPING NEXT TO THE PINHAL-IPUIUNA BATHOLITH (SP-MG) Three groups of granitic rocks outcropping in the neihbourhood of the calc-alkaline Pinhal-Ipuiuna batholith. (BPI) were considered genetically and/or temporally unrelated to the main body of the batholith on the basis of field and petrographic criteria. They correspond to porphyritic biotite monzogranites and granodiorites, equigranular to inequigranular anatectic granites (Pinhal-type granites) as well as hornblende-biotite monzonites that make up a small body named Maravilha Monzonitic Massif. The porphyritic biotite granitoids are heterogeneous, foliated rocks distinguished from the BPI granitoids by the lack of amphibole and close association with the anatectic equigranular granites. The latter, in turn, are represented by fineto medium-grained monzoand syenogranites strongly heterogeneous in the outcrop; they are closely related to regional migmatites and are interpreted as results of crustal anatexis. The Maravilha Monzonitic Massif is formed by mediumto coarse-grained hornblende-biotite monzonites and biotite quartz monzonites showing well-developed flow foliation. The porphyritic biotite granitoids are chemically distinguished from similar rocks of the BPI only due to their higher Fe and lower K contents, as well as higher A/CNK. Remarkable similarities in terms of trace elements, including the REE, suggest that some granodiorites migth have some kind of relationship with the BPI, although if this is the case they should be derived from different (less potassic) magmas. On the other hand, textural and field aspects suggest that any genetic relationships between the monzogranites that are predominam in this unity and the BPI are highly improbable. The Pinhal-type granites are easily distinguished from differentiated granites from the BPI by their lower Ca, Sr and Cr, higher Fe, Zr and Rb, as well as more fractioned REE patterns. Two subgroups of Pinhal-type granites that are recognized on the basis of their acessory minerais are chemically different; the subgroup showing allanite and titanite as important accessories has more fractioned REE patterns displaying pronounced negativo Eu anomalies, besides higher Zr contents and lower mg#. Such contrasts may reflect differences in protoliths, in temperaturas of melting and/or in mechanisms of melt extraction. The Maravilha monzonites are quartz-poor felsic rocks of uncertain stratigraphic position with a very peculiar chemical signature, specially their strongly fractionated REE patterns and high Ba, Sr and Zr contents.