Geochemistry of hybrid granitoid rocks and of their biotites from central northern Portugal and their petrogenesis |
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| Authors: | AMR Neiva |
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| Institution: | Department of Mineralogy and Geology, University of Oporto, 4000 Oporto, Portugal |
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| Abstract: | In the area of Franzilhal, S. Lourenço and Pombal, central northern Portugal, occur rocks ranging from hornblende-biotite tonalite to muscovite-biotite granite. The distribution of some major and trace elements in the rocks and in the biotite shows an apparent calc-alkalic trend of differentiation. The behaviour of K, Li, Rb, Cs and K/Rb, Cr.103/Fe3+, Ni.103/Mg, Li.103/Mg, Cs.103/K ratios of the rocks and of F of biotite cannot be explained by mineral fractionation, but can be described in terms of hybridization. The tonalite, granodiorites and biotite-muscovite granite are hybrid rocks formed by assimilation of pelitic sediments and dolomitic marbles by the granite magma. The fO2 values range from 10?14.8 to 10?16.3; fO2 attains higher values during the crystallization of hybrid rocks than in the uncontaminated muscovite-biotite granite (GIII) magma. The fH2O and PH2O values generally increase and the fH2O/fHF ratio decreases in the hybrid rocks from tonalite to biotite-muscovite-bearing granodiorite and is smaller than in the uncontaminated granite magma. |
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