An Experimental Study of a Lower Proterozoic A-type Granite from theEastern Amazonian Craton, Brazil

Abstract

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 horblende biotite monzogranite with K (sub 2) O/Na (sub 2) O between 0.8 and 1.5 and FeO (sub t) / (FeO (sub t) + 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 degrees C for various melt H (sub 2) O contents and for both oxidizing and reducing fO (sub 2) [NNO (nickel-nickel oxide)+2.5 and NNO-1.5 on average]. For NNO+2.5 and under H (sub 2) O-rich conditions, ilmenite, clinopyroxene, magnetite and hornblende are near-liquidus phases, followed by plagioclase. The orthopyroxene stability field is restricted to high temperatures and H (sub 2) O 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 H (sub 2) O content to between 4.5 and 6.5 wt %. Plagioclase cores crystallized from 870 to 720 degrees C. Clinopyroxene, amphibole and biotite Fe/(Fe+Mg) values suggest fO (sub 2) around NNO+0.5 during crystallization of the granite. The demonstration of relatively hydrous conditions and oxidizing fO (sub 2) for the Jamon granite stresses the diversity of A-type magmas in terms of H (sub 2) O contents and redox states. The Jamon granite was most probably generated from oxidized Archaean igneous rocks of mafic-intermediate composition.