Evolução geológica arqueana do extremo sul do Cráton São Francisco na região da Serra da Babilônia, Bom Sucesso - MG

Abstract

This work deals with the Archean magmatic and metamorphic evolution of the southernmost São Francisco craton (SFC) in the region of Serra da Babilônia (MG-Brazil) and is presented as two papers. The first one can be summarized as follows: to the west of the Bom Sucesso city, occur metamorphosed neoarchean lithotypes ranging from ultramafic to mafic terms, enderbitic to charnockites, and even granites. The petrographic and geochemical data treatment of these litothypes identified five distinct geological units: (I) banded biotite-muscovite gneiss, found in the southeast portion of the studied area, being the oldest rock in the region (~ 3000 Ma); (II) Capela São Francisco metagranitoid, related to 2700 Ma granitoids found in the adjacent southern portion of the SFC. It is peraluminous, high K, low Ca. This unit is derived from partial melting of pelitic crust represented by gneisses of the unit I; (III) 2718-2708 Ma Rio do Amparo-Bom Sucesso metacharnockites to metacharnoenderbites derived from mantle and crustal sources, metaluminous and calc-alkaline; (IV) 2730-2660 Ma Babilônia metaenderbites to metacharnoenderbites, Na-rich metaluminous, ranging from calcic to calc-alkaline. Its origin is linked to the partial melting of mantle sources. The presence of orthopyroxene replacing to amphibole in the units III and IV suggests that the crystallization/recrystallization of these rocks occurred under granulite facies, with retrometamorphism in amphibolite facies; (V) 2660 Ma metanorites/metagabronorites and peridotitic metaultramafics, the first ones being the product of magmatic differentiation of the latter. They have similar REE patterns and the same spatial arrangement within the study area. Although recrystallized, they locally keep preserved igneous textures. Three generations of not Archean mafic dykes cut these units. This association records the neoarchean magmatic and metamorphic evolution of a significant portion from southern SFC during late-collisional stage of the Rio das Velhas event and has similar age and tectonic context described in several cratons around the world. The second article discusses the U-Th-Pb chemical dating by electron microprobe held in monazite crystals with phosphate-silicate reaction coronas found in the Babilônia metacharnoenderbite and the relations between age and the coronas. This second article can be summarized as follows: one Babilônia metacharnoenderbite crystallized and metamorphosed under granulite facies around 2700 Ma keeps monazite crystals with apatite, allanite and epidote coronas arising due to the interaction of monazite with metamorphic fluids containing Ca, Fe, Si and Al. By electron microprobe, an average chemical age of 2657 ± 36 Ma was obtained. It is suggested that it corresponds to the age of amphibolite facies retrometamorphism developed during the Neoarchean late-collisional period of the Rio das Velhas event, time during which the phosphate-silicate reaction coronas would have developed contemporarily to the growth of amphibole and biotite in such rock. This presentation was established in function of the subject of each article and both have results that are required to the concluding remarks of the matter proposed to this dissertation.