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.