CARACTERIZAÇÃO GEOTECTÔNICA DO SETOR SETENTRIONAL DO CINTURÃO RIBEIRA: EVIDÊNCIA DE ACRESÇÃO NEOPROTEROZÓICA NO LESTE DE MINAS GERAIS, BRASIL.

Abstract

The early neoproterozoic evolution of eastern Minas Gerais is characterized by a history of a continental margin activity, including the accretion of suspect terranes. The Manhuaçu Terrane is one of those and is represented by a granitic continental plutonic arc and terrigeneous metasediments that reflect a continental margin. A metasedimentary gneiss belt at this margin with shallow to deep marine clastic lithologies as well as metavulcanic and metaplutonic mafic rocks was interpreted as an extensive tectonic segment with suspect development in a back-arc setting. Fragments of a volcanic arc are identified and interpreted as an evidence for a probable island-arc domain. In the studied region the Juiz de Fora and Paraiba do Sul Complexes are domains of these terranes.

The distribution of magmatism in the studied region shows from west to east three different suites : 1) tholeiitic, 2) a medium to high-K cal-alkalic and 3) a high-K calc-alcalic/shoshonitic(?) magmatism which attests the evolution of early magmatism arcs (volcanic and plutonic) initially relates to ocean-plate subduction, followed by a continent-continent collision.

Petrological, structural and geochemical data of mesoproterozoic/neoproterozoic metamorphic and magmatic suites of rocks are in agreement with the development of an accretionary orogeny. After a vulcanic-(island-arc) and a back-arc basin formation (by rifting of a continental margin with no spreading) in connection with eastward subduction, the island arc was accreted to a continental margin. Further subduction beneath it and a mechanism of flattening of the slab during the subduction process was responsible for the development of as granitic continental plutonic-arc (Andean-type batholith) eastwardly in a compressional setting. Continental plates became sutured and all intervening oceanic lithosphere was subducted beneath one of the converging masses. This resulted in the accretion of the Manhuaçu Microplate to the São Francisco Craton Domain. Plates continued to converge and the inversion of subduction polarity occurs resulting in a new subduction system (westward subduction) to the back of the Manhuaçu Microplate, in a easternward of the Espírito Santo state, with the establishment of a new magmatic arc of late neoproterozoic to eopaleozoic age. In this paper, only the probable early neoproterozoic volcanic (tholeiitic magmatism transitional between N-type MORB and island are basalts) and the plutonic (early continental calc-alkalic magmatism) arcs characterized by rocks with a very particular geochemistry and the back-arc basin setting will be discussed.

Two alternative hypotheses to explain the evolution of these terranes may be postulated: 1) an island arc orogeny related to a westward subduction followed by a cordillerean type orogeny. With the advance of the island-arc and continent with offshore sediments, in  different plates, a collision between these domains occurred. The old subduction zone was replaced by another one, eastward directed and the development of a continental magmatic arc occurred. Continental plates became sutured and all intervening oceanic lithosphere was subducted beneath one of the converging masses. Against this model are the presence of metavulcanic basic and intermediate rocks intercalated with marine and continental margin; 2) an extensional ensialic setting (aborted marginal basin) in the continental margin formed during the opening of a rapidly subsiding basin, with moderately rapid mantle upwelling, pronounced thinning of the continental crust and slight crustal contaminanton, without formation of oceanic crust or an island-arc may explain the association of basic metavulcanic rocks with marine and continental metasediments. Here, the low potassium contents of a few studied metavulcanic basic rocks and theire oceanic tholeiitic affinity are not well in agreement with this setting where continental basalts (flood basalts), or rocks ( basalts or basaltics andesites) belong to the high-K calc-alkaline series are expected to occur. Nevertheless, our evidences are not unequivocal enough for theire disapprovals.