Geologia, petrografia e susceptibilidade magnética do Plúton Granítico Picuí (PB), Domínio São José do Campestre, Província Borborema

Abstract

The Picuí Granitic Pluton (PGP) corresponds to a batholithic body (312 km2) in the Paraíba State, intrusive in orthogneisses of the Serrinha-Pedro Velho Complex, in the São José do Campestre Domain, Borborema Province. This work presents the geological and petrographic characterization of the PGP based on an integrated study involving geological mapping, structural, petrographic, modal and magnetic susceptibility (MS) data. The PGP consists of granitic rocks grouped into two petrographic facies: (1) the main inequigranular to porphyritic facies, which includes alkali-feldspar granites, quartz syenite and syenogranites; and (2) the equigranular fine- to medium grained facies toward the pluton borders, which comprises alkali-feldspar granites and monzogranites. Biotite and hornblende are the main mafic minerals, whereas zircon, magnetite, titanite, apatite, and allanite are relevant accessories in both facies. The MS in the inequigranular to porphyritic granites shows a normal distribution and an average of 5.22 x 10-3 SI, slightly higher than the average value for the equigranular fine- to medium grained facies (4.96 x 10-3 SI) which, on the other hand, shows higher standard deviation due to the positively skewed distribution of the MS, eventually with anomalously high values (>20 x 10-3 SI). In the latter case, such values may also be associated to subsolidus hydrothermal alteration processes in the PGP rocks, which were also responsible for the sericitization and saussuritization of feldspars and the destabilization of mafic minerals into the paragenesis titanite+chlorite+muscovite+magnetite. Textural evidence suggests that fractional crystallization was an important mechanism of magmatic evolution in the PGP, under relatively oxidized conditions. On the other hand, the common presence of xenoliths and megaxenoliths (roof pendants?) from the surrounding orthogneisses may suggest, at least locally, the influence of crustal contamination processes. The modal data obtained for the PGP indicate that its rocks can be classified as type-A granites, as for other porphyritic plutons and batholiths in the region. According to the current proposal of magmatic suites for the granitogenesis in the São José do Campestre and Rio Piranhas-Seridó Domains, the PGP falls into the equigranular high-K calcium-alkaline suite. In view of the predominance of inequigranular to porphyritic granites in this pluton, it is suggested that the PGP should be rather placed into the porphyritic high-K calcium-alkaline suite (Itaporanga type).