Geochemistry of the granitoid rocks and their minerals from Lixa do Alvão-Alfarela de Jales-Tourencinho (Vila Pouca de Aguiar, northern Portugal)

Abstract

Several types of Hercynian syntectonic peraluminous granites containing biotite and muscovite and post-tectonic sub-aluminous to peraluminous biotite granite with inclusions of biotite granodiorite occur in the area of Lixa do Alvão-Alfarela de Jales-Tourencinho. NbY, TaYb, Rb(Yb + Ta) and Rb(Y + Nb) diagrams of rocks, some variation diagrams of rocks and biotites, REE patterns of rocks and thermodynamic conditions of crystallization show distinction between these two groups. The oldest syntectonic granite (320 ± 12 Ma) is a fine-grained muscovite-biotite granite corresponding to the Erzgebirgian Hercynian event. The other syntectonic granites range from two-mica granite (309 ± 3 Ma, representing the Late Wichita Hercynian event) to muscovite granite with tourmaline. Variation diagrams of major and trace elements of rocks, biotites and muscovites show fractionation trends for the syntectonic granites ranging from two-mica granite to muscovite granite with tourmaline. The oldest granite does not belong to them. This granite and the least silicic samples of the two-mica granite correspond to two distinct magmas originated by partial melting of metasedimentary materials and emplaced at 3.5-3 kbar, 690°C, f O 2 10 −18–10 −17 bar at 320 ± 12 and 309 ± 3 Ma ago, respectively. Least-squares analysis of major elements and modelling of trace elements indicate that the most silicic samples of the two-mica granite were derived from its magma by fractional crystallization of plagioclase, quartz and biotite. However, the great influence of volatiles producing metasomatic effects prevented testing whether the most evolved later syntectonic granites were derived from the same magma by fractional crystallization. These granites were completely crystallized at ∼ 600-440°C and metasomatized at ∼ 400°C. The post-tectonic granite is also crust anatectic and its magma was emplaced at pressures of > 3.5 kbar, 710°C and f O 2 of 10 −16 bar. Its granodioritic inclusions are hybrid rocks and the result of assimilation of country rocks by the host granite magma. Inclusions and granite were completely crystallized at ∼ 700-470°C and metasomatized at 440°C.