The Jamon granite is representative of the Lower Proterozoic (1·88 INTRODUCTION Ga) oxidized A-type granites of the eastern part of the Amazonian Proterozoic anorogenic granites are found in several craton. The dominant facies is a metaluminous to slightly percratonic areas, such as in the Fennoscandian shield aluminous hornblende biotite monzogranite with K2O/Na2O be(Nurmi & Haapala, 1986; Haapala & Ramo, 1990; Ramo tween 0·8 and 1·5 and FeOt/(FeOt + MgO) between 0·8 and & Haapala, 1991), in North America (Bridgwater & 0·9. In contrast to many other A-type granites, the Jamon granite Windley, 1973; Barker et al., 1975; Van Schmus et al., is characterized by early hornblende crystallization and the presence 1987; Anderson & Bender, 1989; Gower et al., 1990; of magnetite. Crystallization experiments were performed on glass Emslie, 1991; Emslie & Stirling, 1993) and Australia at 300 MPa between 700 and 900°C for various melt H2O (Wyborn et al., 1992; Pollard & Mitchell, 1995). They contents and for both oxidizing and reducing fO2 [NNO (nickel– are widespread in several tectonic provinces of the Amnickel oxide) + 2·5 and NNO – 1·5 on average]. For NNO azonian craton, which is now recognized as one of + 2·5 and under H2O-rich conditions, ilmenite, clinopyroxene, the world’s major provinces of anorogenic magmatism magnetite and hornblende are near-liquidus phases, followed by (Dall’Agnol et al., 1994, 1999a; Bettencourt et al., 1995). plagioclase. The orthopyroxene stability field is restricted to high Anorogenic Amazonian granites have ages ranging from temperatures and H2O contents in the melt <5 wt %. In contrast, ~1·9 to 1·0 Ga (Machado et al., 1991; Dall’Agnol et al., for NNO – 1·5, magnetite and titanite are absent and orthopyroxene 1994; Bettencourt et al., 1995) and intrude either Ar(never observed in the granite), clinopyroxene and ilmenite are the chaean or early to middle Proterozoic sequences. They liquidus phases. Conditions of crystallization of amphibole, magare petrologically and geochemically diverse, with rock netite and plagioclase constrain the initial melt H2O content to types similar to those found in rapakivi granites from the between 4·5 and 6·5 wt %. Plagioclase cores crystallized from Fennoscandian shield (Haapala & Ramo, 1992; Ramo 870 to 720°C. Clinopyroxene, amphibole and biotite Fe/(Fe + & Haapala, 1995). The Amazonian anorogenic granites Mg) values suggest fO2 around NNO+ 0·5 during crystallization are classified as A-type (Dall’Agnol et al., 1994, 1999a, of the granite. The demonstration of relatively hydrous conditions 1999b). Many granite complexes are mineralized (Sn, W, and oxidizing fO2 for the Jamon granite stresses the diversity of REE, Zr, F, Nb, Be, Ta), and the tin deposits associated A-type magmas in terms of H2O contents and redox states. The with these granites are largely responsible for Brazil Jamon granite was most probably generated from oxidized Archaean igneous rocks of mafic–intermediate composition. becoming a major tin producer in the last 20 years. Given the importance of the province, large time span covered, variety of rock types and the significant mineral resources associated, there is clearly a need for a better