Geochemistry of the granites and their minerals from Paredes da Beira-Penedono, northern Portugal

Abstract

Seven types of Hercynian peraluminous muscovite-biotite granites (T 1-T 5) and muscovite granites (T 6 and T 7) occur in the Paredes da Beira-Penedono area. Microscopically they show some small metasomatic effects (e.g., microclinization of plagioclase, sericitization of feldspars, small chloritization of biotite, small amounts of late-stage white mica and kaolinization of microcline). Three different types of white mica (muscovite, sericite and late-stage white mica) can be chemically distinguished in a TiMgNa diagram. Variation diagrams of major and trace elements and elemental ratios of granites and muscovites show trends of fractionation from the muscovite-biotite granites to the muscovite granites. Least-squares analysis of major elements and modelling of trace elements indicate that the muscovite granite T 7 might have been derived from the muscovite-biotite granite magma T 1 by fractional crystallization of quartz, K-feldspar, plagioclase, muscovite, biotite and ilmenite. Elemental variation diagrams of granites, biotites and muscovites also suggest trends of fractionation: (a) from the muscovite-biotite granite T 1 to the muscovite-biotite granite T 4; (b) in the muscovite-biotite granite T 5; (c) in the muscovite granite T 6. However, the small metasomatic effects make it difficult to test fractional crystallization. The granite T 5 has a large range of composition from the least to the most silicic samples, similar to the variation of compositions of the granites in the sequence T 1 → T 4. The granite T 1 and the least silicic samples of the granite T 5 would have their origin in two distinct pulses of granite magma, and probably have originated by partial melting of metasedimentary materials. Emplacement of granite magmas took place at ∼ 700°C and 4 kbar. They were completely crystallized at ∼ 450°C and slightly metasomatically altered at ∼ 370°C. log( f H 2 O f HF ) is smaller in the muscovite granites than in the muscovite-biotite granites. log( f H 2 O f HCl ) increases and log( f HCl f HF ) and log( f H 2 O f HF ) decrease in the sequence T 1 → T 4 of muscovite-biotite granites.