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Journal of Petrology, Volume 40, Issue 11: November 1999.
The Jamon granite is representative of the Lower Proterozoic (1·88 Ga) oxidized A-type granites of the eastern part of the Amazonian craton. The dominant facies is a metaluminous to slightly peraluminous hornblende biotite monzogranite with K2O/Na2O between 0·8 and 1·5 and FeOt/(FeOt + MgO) between 0·8 and 0·9. In contrast to many other A-type granites, the Jamon granite is characterized by early hornblende crystallization and the presence of magnetite. Crystallization experiments were performed on glass at 300 MPa between 700 and 900°C for various melt H2O contents and for both oxidizing and reducing fO2 [NNO (nickel-nickel oxide) + 2·5 and NNO - 1·5 on average]. For NNO + 2·5 and under H2O-rich conditions, ilmenite, clinopyroxene, magnetite and hornblende are near-liquidus phases, followed by plagioclase. The orthopyroxene stability field is restricted to high temperatures and H2O contents in the melt <5 wt %. In contrast, for NNO - 1·5, magnetite and titanite are absent and orthopyroxene (never observed in the granite), clinopyroxene and ilmenite are the liquidus phases. Conditions of crystallization of amphibole, magnetite and plagioclase constrain the initial melt H2O content to between 4·5 and 6·5 wt %. Plagioclase cores crystallized from 870 to 720°C. Clinopyroxene, amphibole and biotite Fe/(Fe + Mg) values suggest fO2 around NNO + 0·5 during crystallization of the granite. The demonstration of relatively hydrous conditions and oxidizing fO2 for the Jamon granite stresses the diversity of A-type magmas in terms of H2O contents and redox states. The Jamon granite was most probably generated from oxidized Archaean igneous rocks of mafic-intermediate composition.
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Pages 1673-1698