O MAGMATISMO GRANÍTICO NEOPROTEROZÓICO DO BATÓLITO PELOTAS NO SUL DO BRASIL: NOVOS DADOS E REVISÃO DA GEOCRONOLOGIA REGIONAL

Abstract

The Pelotas Batholith is an about 40,000 km2 polyphase plutonic complex located in the eastern part of Sul-rio-grandense Shield, southernmost Brazil. It is composed of six granitic suites (Pinheiro Machado, Erval, Viamão, Encruzilhada do Sul, Cordilheira and Dom Feliciano) and one syenitic suite (Piquiri). The batholith shows a compositional evolution from suites dominated by granodiorites and monzogranites with minor tonalites, diorites, and quartz-diorites to those dominated by syenogranites with microcline-ortoclase granites, quartz-syenites, and subordinated syenites. All the suites contain basic and intermediate rocks with mixing and mingling relations indicating they are coeval with the granitic magmatism. The Pinheiro Machado Suite has a medium to high-K nature and ages from 625 Ma to 610 Ma and the Viamão Suite shows high-K calc-alkaline affinity and ages about 600 Ma. The Cordilheira Suite is more peraluminous then the former. The Piquirí Suite resulted from an alkaline satured shoshonitic magmatism at about 610 Ma. Metaluminous and peralkaline alkaline supersatured rocks form the Encruzilhada do Sul Suite, with ages between 600 Ma and 590 Ma, and in granite and rhyolite dikes included in the Don Feliciano Suite, of 595 Ma to 570 Ma. Basement megaxenoliths are widespread in these suites and are similar to the metamorphic rocks found at the western border of this batholith. The Pelotas Batholith contains evidence of two ductile deformational events (D1 and D2 ) and one ruptile (D3 ). Most suites were emplaced in a transpressive regime, during a post-collisional stage, being partially accompanied by oblique convergence and transcurrence of continental plates. Available geocronological data indicate that D1 and D2 have similar age and that the Pelotas Batholith magmatism has ages from 630 Ma to 570 Ma. Granitic magmatism of the batholith shows an important mantle contribution represented by diorites associated to most of the studied suites, which are part of a mafic magmatism The generation and emplacement of the granitic magmatism are related to the evolution of the ductile shear zones. These structures have also influenced the occurrence of mafic magmatism that contributed to increase the geothermal gradients and to promote crustal melting and the generation of granitic rocks in the batholith