Tectonic implications of precambrian Sm-Nd dates from the southern São Francisco craton and adjcent Araçuaí and Ribeira belts, Brazil.

Abstract

The Archean and Paleoproterozoic São Francisco craton of eastern Brazil is surrounded on all sides by Brasiliano (=Pan African) orogens. The N–NE trending orogen that separates the eastern edge of the southern São Francisco craton from the Atlantic coast can be divided into the largely greenschist and amphibolite facies Araçuaı́ belt on the west and the largely granulite facies Ribeira belt on the east. A pronounced linear gravity and magnetic anomaly, the Abre Campo discontinuity, defines the boundary between these two belts. We obtained Sm–Nd mineral ages and whole-rock Sm–Nd model ages for garnet-bearing metamorphic rocks along an E–W transect across the southern São Francisco craton, the southern Araçuaı́ belt, and the Ribeira belt at about latitude 20°S. A recrystallization age of 2.1 Ga from metasediments recrystallized during the development of the classic dome-and-keel province of the southern São Francisco craton (the ‘Quadrilátero Ferrı́fero’) indicates that dome emplacement occurred during the waning stages (extensional collapse) of the Transamazonian collisional orogeny. Seven mineral ages from the southern Araçuaı́ and Ribeira belts date the thermal peak of metamorphism at between 538 and 589 Ma, confirming that these belts were pervasively remobilized during the Brasiliano event. Samples from the Araçuaı́ belt yield either Archean (>2.6 Ga) or Transamazonian (2.1–2.3 Ga) TDM model ages, indicating that the protoliths were either fragments of the São Francisco craton crust or were sediments derived from that craton, which presumably had been stretched to form a thinned continental margin during Meso- and Neo-Proterozoic rifting events. Notably, a mixed meta-pelite and metabasite sequence in the southern Araçuaı́ belt, the Dom Silvério Group, has Transamazonian ancestry and thus may represent oceanic sediments deposited on or east of this stretched margin and then thrust back onto the continent to mark a collisional suture between the São Francisco block and an Archean (?) crustal sliver to the east. The rocks of the Ribeira belt (i.e. the region east of the Abre Campo discontinuity) have younger model ages (TDM=1.6–2.0 Ga), indicating that this belt was not originally part of the São Francisco craton — it may represent an accreted Transamazonian terrane. Therefore, the Abre Campo discontinuity marks an important crustal boundary, possibly a suture. Brasiliano shear zones appear to steepen and root into the Abre Campo discontinuity, suggesting that it formed in Brasiliano time.