GEOCHEMISTRY AND U-Pb GEOCHRONOLOGY OF THE NEOARCHEAN GNEISSES AND PALEOPROTEROZOIC GRANITES FROM JANUÁRIA HIGH: RECORDS OF JUVENILE AND YOUTHFUL EARTH IN THE SÃO FRANCISCO CRATON NUCLEUS (BRAZIL): Geoquímica e geocronologia U-Pb dos gnaisses neoarqueanos e granitos paleoproterozoicos do Alto de Januária: registros da Terra Juvenil e Jovem no núcleo do Cráton São Francisco (Brasil)

Abstract

Within the proposal of secular evolution of the Earth system divided into seven phases: “Proto-Earth” (4.57–4.45 Ga); “Primordial Earth” (4.45–3.80 Ga); “Primitive Earth” (3.8–3.2 Ga); “Juvenile Earth” (3.2–2.5 Ga); “Youthful Earth” (2.5–1.8 Ga); “Middle Earth” (1.8–0.8 Ga); and “Contemporary Earth” (since 0.8 Ga), the 2.61-2.64 Ga High-K, meta- to peraluminous Januária High orthogneisses are inserted in the context of almost rigid behavior of the lithosphere, by the end of the Juvenile Earth. With protolith origin suggestive of intracrustal partial melting of local older TTG (tonalite-trondhjemite-granodiorite), these orthogneisses are the representatives in the São Francisco craton nucleus of the 2.8 Ga to 2.6 Ga potassic rocks found in archean cratons worldwide, and the occurrence of these rocks is a defining characteristic of the Juvenile Earth. Supported on evidence of the plate tectonic activity in Paleoproterozoic, the 2.14-2.19 Ga Januária High biotite granites are inserted in the context of Youthful Earth and they are the record of accretionary orogeny delineated by the edification of magmatic arc at that time. They are calc-alkaline, I-type rocks typical of subduction-related continental magmatic arcs. These biotite granites are probably derived from a High-K mafic source associated with tonalites.ABSTRACT - Within the proposal of secular evolution of the Earth system divided into seven phases: “Proto-Earth” (4.57–4.45 Ga); “Primordial Earth” (4.45–3.80 Ga); “Primitive Earth” (3.8–3.2 Ga); “Juvenile Earth” (3.2–2.5 Ga); “Youthful Earth” (2.5–1.8 Ga); “Middle Earth” (1.8–0.8 Ga); and “Contemporary Earth” (since 0.8 Ga), the 2.61-2.64 Ga High-K, meta- to peraluminous Januária High orthogneisses are inserted in the context of almost rigid behavior of the lithosphere, by the end of the Juvenile Earth. With protolith origin suggestive of intracrustal partial melting of local older TTG (tonalite-trondhjemite-granodiorite), these orthogneisses are the representatives in the São Francisco craton nucleus of the 2.8 Ga to 2.6 Ga potassic rocks found in Archean cratons worldwide, and the occurrence of these rocks is a defining characteristic of the Juvenile Earth. Supported on evidence of the plate tectonic activity in Paleoproterozoic, the 2.14-2.19 Ga Januária High biotite granites are inserted in the context of Youthful Earth and they are the record of accretionary orogeny delineated by the edification of magmatic arc at that time. They are calc-alkaline, I-type rocks typical of subduction-related continental magmatic arcs. These biotite granites are probably derived from a high-K mafic source associated with tonalites. RESUMO - Dentro da proposta da evolução secular do sistema Terra dividido em sete fases: “Proto-Terra” (4,57–4,45 Ga); “Terra Primordial” (4,45–3,80 Ga); “Terra Primitiva” (3,8–3,2 Ga); “Terra Juvenil” (3,2–2,5 Ga); “Terra Jovem” (2,5–1,8 Ga); “Terra Média” (1,8–0,8 Ga); e “Terra Contemporânea” (desde 0,8 Ga), os ortognaisses meta- a peraluminosos de alto-K de 2,61-2,64 Ga do Alto Januária estão inseridos no contexto de comportamento quase rígido da litosfera, no final da “Terra Juvenil”. Com origem de seu protólito sugestiva de fusão parcial intracrustal dos TTG (tonalito-trondhjemito-granodiorito) locais mais antigos, esses ortognaisses são os representantes no interior do cráton do São Francisco das rochas potássicas de 2,8 Ga a 2,6 Ga encontradas em crátons arqueanos em todo o mundo, e a ocorrência dessas rochas é uma característica definidora da “Terra Juvenil”. Apoiados nas evidências da atividade da tectônica de placas no Paleoproterozóico, os biotita granitos de 2,14-2,19 Ga do Alto de Januária estão inseridos no contexto da “Terra Jovem”, são o registro da orogenia acrescionária delineada pela edificação de arco magmático continental nessa época e tratam-se de rochas cálcio-alcalinas do tipo I, típicas de arcos, provavelmente derivadas de uma fonte máfica de alto K associada a tonalitos