Tectonic implications of Precambrian Sm-Nd dates from the southern Sao Francisco craton and adjacent Aracuai and Ribeira belts, brazil

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Precambrian Research
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The Archean and Paleoproterozoic Sao 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 Sao Francisco craton from the Atlantic coast can be divided into the largely greenschist and amphibolite facies Aracuai bell 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 Sao Francisco craton, the southern Aracuai belt, and the Ribeira belt at about latitude 20 degrees S, A recrystallization age of 2.1 Ga from metasediments recrystallized during the development of the classic dome-and-keel province of the southern Sao Francisco craton (the 'Quadrilatero Ferrifero') indicates that dome emplacement occurred during the waning stages (extensional collapse) of the Transamazonian collisional orogeny. Seven mineral ages from the southern Aracuai 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 Aracuai belt yield either Archean (> 2.6 Ga) or Transamazonian (2.1-2.3 Ga) T-DM model ages, indicating that the protoliths were either fragments of the Sao 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 Aracuai belt, the Dom Silverio 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 Sao 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 (T-DM = 1.6-2.0 Ga), indicating that this belt was not originally part of the Sao 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. (C) 2000 Elsevier Science B.V. All rights reserved.


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