Title: Geochemistry of biotites from granitic rocks, Northern Portugal
Abstract: The biotites from a series of rocks ranging in composition from tonalite to granite have been analysed for both major and trace elements. The relations between chemical composition and paragenesis of the biotites are studied. Most biotites co-exist with potassium feldspar and ilmenite. Variations in composition can be correlated with the occurrence of amphibole, primary muscovite and aluminosilicates in the rocks. Variation diagrams of the trace element contents and element ratios of biotite are compared to those of the host rocks. Fractionation of elements can be defined more accurately as the influence of other mineral phases is eliminated. Variations in the proportions of the octahedrally co-ordinated Al, Ti and Fe3+ are correlated with the conditions of crystallization and comparisons made with biotites from other suites of calc-alkali rocks. In the light of the experimental data available, the petrographic observations and the chemical data it is apparent that biotites crystallized from systems in which fO2 was buffered, its values remaining close to that of the buffer FMQ. From the same data, a temperature of 800°C for fO2 = 10−14 to 10−15 bars is deduced as prevalent during the crystallization of the tonalites while for the granites, at a temperature of crystallization of 680°C, fO2 = 10−16 to 10−18 bars. A calc-alkali trend of fractionation is therefore apparent with decreasing fO2 while fH2O2 remains relatively high.
Publication Year: 1973
Publication Date: 1973-07-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 127
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