CRUSTAL EVOLUTION IN THE EARLY ARCHEAN OF SOUTH-AMERICA - EXAMPLE OF THE SETE VOLTAS MASSIF, BAHIA STATE, BRAZIL

The Sete Voltas massif (Sao Francisco craton, Brazil) appears as a com posite crustal segment built in at least three successive accretional events dated by Rb-Sr, Pb-207/Pb-206 monozircon and U-Pb SHRIMP method s. (a) At ca 3.4 Ga, generation and emplacement of magmatic precursors to the old grey gneisses. These are the oldest rocks so far recognize d in South America. Their composition is typical of Archaean TTG (Tona lite, Trondhjemite and Granodiorite) and geochemical modelling indicat es that they were produced by partial melting of an Archaean tholeiite , leaving a hornblende garnet residue. (b) Between 3.17 and 3.15 Ga, y ounger grey gneisses and porphyritic granodiorites intruded the old gr ey gneisses. Their geochemical composition is clearly different from t hat of typical TTG and geochemical modelling shows that they were prod uced by partial melting of an older continental crust. (c) Grey granit e dykes emplaced at ca 2.6 Ga, during a late-stage magmatic event in t he Sete Voltas massif. All the units belonging to the Sete Voltas mass if yield homogeneous T-DM ages at 3.66 Ga which are interpreted as ref lecting derivation from older protoliths, or rather their contaminatio n 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 ne vertheless reported a single zircon core from Sete Voltas giving a Pb- 207/Pb-206 age of 3.473 +/- 0.008 Ga. The present study, together with some other recent data (Mougeot et al., 1995; Santos Pinto et al., 19 95a) indicates that the 3.4-Ga-old South American continental crust, u ntil now considered as of very small geographical extent, in fact made up a true continental block corresponding in size to at least the who le present-day Gaviao block. Major and trace element petrogenetic mode lling for the young grey gneisses and the porphyritic granodiorites in dicates that the precursor magmas are derived from the melting of pre- existing crustal rocks similar to the old grey gneisses. This conclusi on is supported by the intense migmatization of the old grey gneisses which took place before the intrusion of the younger intrusives. The c alculated residual mineral assemblage in equilibrium with the magmatic liquid is quartz + hornblende + plagioclase, a condition that is achi eved between P = 10 kbar, T approximate to 800 degrees C, < 5% weight H2O, or P=15 kbar, T approximate to 700 degrees C, similar to 15% weig ht H2O (Johnson and Wyllie, 1988). This suggests that the hydrous melt ing 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 o f the Archaean crust. This estimate, as well. as the development of an intense migmatization and the strong foliation in the old grey gneiss es, 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 mod ern types of plate tectonic mechanisms operated in the early Archaean in the studied terrain. (C) 1997 Elsevier Science B.V.