Hilton Tulio Costi

Rapakivi granites from Brazil and adjacent areas

Abstract A review of the rapakivi magmatism in Brazil and adjacent areas demonstrates that the dominantly felsic anorogenic complexes of the Amazonian craton can be considered as rapakivi suites. They are found in almost all provinces of the craton and are concentrated in the timespan covering the end of the Palaeoproterozoic until the beginning of the Neoproterozoic. The Mesoproterozoic tin granites of the Goias province, correlated with the anorogenic series of the Amazonian craton, are also included in the rapakivi series. Both are correlated with the Laurentia–Baltica rapakivi suites. Neoproterozoic rapakivi granites and mangerite–charnockite series have been identified in post-collisional tectonic settings related to the Brasiliano event in southeastern Brazil. Most of the rapakivi suites of the Amazonian craton and those of Goias are considered as anorogenic, but probably there are also rapakivi suites associated with post-collisional settings, such as those formed during the Brasiliano event. The latter are possibly related to anatexis of the lower crust associated with crustal thickening. The Precambrian rapakivi granites of South America display A-type and within-plate geochemical signature. They are generally metaluminous to slightly peraluminous, but peralkaline rocks are also found. Basic rocks and charnockitic series coeval with the rapakivi granites are apparently less frequent than in North America. Massif-type anorthosite is very rare, AMCG suites have been found only in the Roraima State of Brazil. Mangerites and charnockites, associated and contemporaneous with the rapakivi granites, have been described in Rondonia, but anorthositic complexes are lacking. Typical wiborgite and pyterlite are found in the Rondonia and Roraima regions in Brazil and in Venezuela (Parguaza Granite). In other regions, medium, even-grained granites are largely dominant. The general picture is thus similar to that observed in the AMCG suites of North America, differing in some degree from the classic Finnish rapakivi granites which display wiborgitic and pyterlitic textures more frequently. The most striking difference between the rapakivi granites of South America and their counterparts of Laurentia–Baltica is that the former are very rich in tin deposits, with associated Y, REE, Th, F (cryolite), Zr and In, while the latter are relatively poor in them. The study of rapakivi series in South America and more particularly in the Amazonian craton will be extremely useful for a better understanding of their petrogenesis and tectonic evolution. The rapakivi distribution in the craton demonstrates that the suggested tectonic provinces should be re-evaluated. Furthermore, the tectonic evolution of the Amazonian craton seems to differ significantly from that of the Sao Francisco craton of Brazil and the Congo–Kasai craton in Africa.

Petrography, magnetic susceptibility and geochemistry of the Rio Branco Granite, Carajás Province, southeast of Pará, Brazil

The Paleoproterozoic Rio Branco stock intrudes the RESUMO: Petrografia, suscetibilidade magnetica e geoquimica do Archean Cruzadao biotite monzogranite. It occurs west of the Canaa dos Carajas city, close to the Sossego copper mine in the Carajas Prov- ince. It is constituted by undeformed and isotropic, hololeucocratic syenogranites, showing equigranular texture. The granite is formed essentially by perthitic alkali feldspar, quartz, and plagioclase, with additional variable amounts of chloritized biotite. Fluorite, allanite, and zircon are common accessory minerals, whereas pyrite and chal- copyrite are scarces. Albitization and subordinate greisenization are the main alteration processes that affected the granite. The secondary mineralogy is represented by albite, fluorite, topaz, chlorite, musco- vite, siderophyllite, and iron oxides. Magnetic susceptibility values are systematically low and vary between 1.3 x 10 -5 a 6.9 x 10 -4 (SI). The Rio Branco Granite is metaluminous to peraluminous and shows high FeOt/(FeOt + MgO). It has geochemical affinities with ferroan, reduced A2-subtype granites. The rare earth element (REE) patterns are flat with little heavy REE fractionation and show accentuated negative Eu anomalies (Eu/Eu* = 0.08 – 0.13). These characteristics are typical of evolved granites, derived from volatile-enriched liq- uids responsible by the intense subsolidus hydrothermal alteration. The dating of the Rio Branco granite did not give conclusive results, but the obtained data, associated with geological evidence, suggest that it is of Paleoproterozoic age. The comparison between the Rio Bran- co granite and Paleoproterozoic A-type suites of the Carajas Province suggests that the Rio Branco granite has more affinity with the Velho Guilherme suite and, in lesser degree, with the Serra dos Carajas suite. On the other hand, is clearly different from oxidized A-type granites of the Jamon suite. Despite its similarities with tin-specialized gran- ites, the Rio Branco Granite is not associated with tin mineralizations.