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 Goiás 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 Goiás 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 Rondônia, but anorthositic complexes are lacking. Typical wiborgite and pyterlite are found in the Rondônia 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 São Francisco craton of Brazil and the Congo–Kasai craton in Africa.