Geochemistry of biotites from granitic rocks, Northern Portugal

摘要: The biotites from a series of rocks ranging in composition tonalite to granite have been analysed for both major and trace elements. The relations between chemical paragenesis the are studied. Most co-exist with potassium feldspar ilmenite. Variations can be correlated occurrence amphibole, primary muscovite aluminosilicates rocks. Variation diagrams element contents ratios biotite compared those host rocks. Fractionation elements defined more accurately as influence other mineral phases is eliminated. Variations proportions octahedrally co-ordinated Al, Ti Fe3+ conditions crystallization comparisons made suites calc-alkali rocks. In light experimental data available, petrographic observations it apparent that crystallized systems which fO2 was buffered, its values remaining close buffer FMQ. From same data, temperature 800°C = 10−14 10−15 bars deduced prevalent during tonalites while granites, at 680°C, 10−16 10−18 bars. A trend fractionation therefore decreasing fH2O2 remains relatively high.