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Abstract

The Sete Voltas massif (São Francisco craton, Brazil) appears as a composite crustal segment built in at least three successive accretional events dated by RbSr, 207Pb206Pb monozircon and U-Pb SHRIMP methods. (a) At ca 3.4 Ga, generation and emplacement of magmatic precursors to the old grey gneisses. These are the oldest rocks so far recognized in South America. Their composition is typical of Archaean TTG (Tonalite, Trondhjemite and Granodiorite) and geochemical modelling indicates that they were produced by partial melting of an Archaean tholeiite, leaving a hornblende garnet residue. (b) Between 3.17 and 3.15 Ga, younger grey gneisses and porphyritic granodiorites intruded the old grey gneisses. Their geochemical composition is clearly different from that of typical TTG and geochemical modelling shows that they were produced by partial melting of an older continental crust. (c) Grey granite dykes emplaced at ca 2.6 Ga, during a late-stage magmatic event in the Sete Voltas massif.All the units belonging to the Sete Voltas massif yield homogeneous TDM ages at 3.66 Ga which are interpreted as reflecting derivation from older protoliths, or rather their contamination by older pre-existing continental crust. It is tentatively proposed that a 3.66-Ga-old continental crust existed prior to the emplacement of the older grey gneisses. Although such crustal material is unknown at the present day in South America, Nutman and Cordani (1993) have nevertheless reported a single zircon core from Sete Voltas giving a 207Pb206Pb age of 3.473 ± 0.008 Ga. The present study, together with some other recent data (Mougeot et al., 1995; Santos Pinto et al., 1995a) indicates that the 3.4-Ga-old South American continental crust, until now considered as of very small geographical extent, in fact made up a true continental block corresponding in size to at least the whole present-day Gavião block.Major and trace element petrogenetic modelling for the young grey gneisses and the porphyritic granodiorites indicates that the precursor magmas are derived from the melting of pre-existing crustal rocks similar to the old grey gneisses. This conclusion is supported by the intense migmatization of the old grey gneisses which took place before the intrusion of the younger intrusives. The calculated residual mineral assemblage in equilibrium with the magmatic liquid is quartz + hornblende + plagioclase, a condition that is achieved between P = 10 kbar, T ≈ 800°C, <5% weight H2O, or P = 15 kbar, T ≈ 700°C, ∼15% weight H2O (Johnson and Wyllie, 1988). This suggests that the hydrous melting of the old grey gneisses occurred at ca 3.17 Ga at depths of ca 30–45 km, thus giving a minimum estimate of the thickness of this part of the Archaean crust. This estimate, as well as the development of an intense migmatization and the strong foliation in the old grey gneisses, can be interpreted as classical features of collisional thickening. Taken with the existence of horizontal tectonics in Archaean cratons older than 3.0 Ga, such a conclusion indicates that, more or less modern types of plate tectonic mechanisms operated in the early Archaean in the studied terrain.