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U-Pb data for granulites of the Anápolis-Itauçu complex: evidence for two high-grade events in the Brasília belt

U-PB DATA FOR GRANULITES OF THE ANÁPOLIS-ITAUÇU COMPLEX: EVIDENCE FOR TWO HIGH-GRADE EVENTS IN THE BRASÍLIA BELT

M

REINHARDT A. FUCK

Instituto de Geociências, Universidade de Brasília, Brasília.

The Tocantins Province represents a large Neoproterozoic orogen in central Brazil. The Brasília Belt consitutes the eastern half of the province, along the western margin of the São Francisco Craton. High-grade rocks occur in three mafic-ultramafic complexes (Barro Alto, Niquelândia and Cana Brava), as well as in the Anápolis-Itauçu Complex (AIC), in the central-southern part of the belt.

The AIC includes: (i) orthogranulites represented by mafic-ultramafic complexes, charnockites, tonalitic gneisses, and (ii) supracrustal granulites including sillimanite-garnet gneisses, marbles and quartzites. Narrow volcano-sedimentary belts made of amphibolites and micaschists are also recognized. Intruded into the high grade rocks are a large number of peraluminous granites, some of which also display high grade mineral assemblages. The rocks of the AIC have been traditionally been interpreted as part of the ancient (Archaean/Paleoproterozoic) sialic basement of the Brasília Belt sediments.

The studies have demonstrated that the high-grade metamorphic event which affected rocks of the mafic-ultramafic complexes occurred at ca. 760-780 Ma (U-Pb zircon and Sm-Nd garnet ages). Preliminary Sm-Nd data for the AIC granulites, on the other hand, indicate that the metamorphic mineral assemblages were formed at ca. 620 Ma.

New U-Pb zircon ages for the AIC rocks indicate that: (i) the granites crystallized at ca. 630-640 Ma (U-Pb ages of igneous zircon crystals), and (ii) the metamorphism happened roughly at the same time (ca. 630 Ma).

The data available suggest, therefore, that two events of high-grade metamorphism took place in the Brasília Belt during the Neoproterozoic. The earlier event (ca. 760-780 Ma) is well documented in granulitic rocks of the northern segment of the belt (Barro Alto, Niquelândia and Cana Brava complexes), whereas the younger (ca. 630 Ma) is registered in granulites of the AIC, in the southern part of the Brasília Belt. The 760-780 Ma event is preliminarily interpreted as the result of the collision between the Goiás Magmatic Arc and the western margin of the São Francisco continent, and the ca. 630 Ma as indicative of final ocean closure. (May 19, 2000)

Publication Dates

  • Publication in this collection
    05 Jan 2001
  • Date of issue
    Dec 2000
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