Original paper

Geochemistry of granitic aplite-pegmatite sills and their minerals from Arcozelo da Serra area (Gouveia, central Portugal)

Neiva, Ana M.R. Gomes

Abstract

Granitic aplite-pegmatite sills intruded a granodiorite-granite and a biotite ≈muscovite granite from Arcozelo da Serra (Gouveia, Portugal). A muscovite > biotite granite also crops out in the area. Variation diagrams of major and trace elements of the rocks show fractionation trends for a) granodiorite-granite and muscovite > biotite granite; b) biotite≈muscovite granite and aplite-pegmatite sills. REE patterns and δ18O of rocks, anorthite contents of plagioclases, Ba contents of potash feldspars, major elements and Li of biotites and muscovites confirm the two series. Least squares analysis of major elements and modelling of trace elements indicate that aplite-pegmatite sills were derived from biotite ≈ muscovite granite magma by fractional crystallization of quartz, plagioclase, potash feldspar and biotite. This mechanism is responsible for the Sn enrichment of aplite-pegmatite sills and Sn is retained in micas. Electron microprobe analyses of columbite-tantalite crystals from aplite-pegmatite sills show oscillatory, progressive and reverse zonings, which are characterized by the behaviours of eight elements and Mn/(Mn+Fe) and Ta/(Ta+Nb) ratios. Oscillatory zoning is mainly attributed to faster crystal growth than Nb, Ta, Fe and Mn can diffuse through liquid, while reverse zoning is due to nucleation and growth of evolved oxide cores and back-reaction of them with the more primitive bulk magma. Other samples of aplite-pegmatite sills show late zoned micas, consisting mainly of a Li-bearing muscovite core and a composition between zinnwaldite and trilithionite for the rim. However, alternating compositions of these two micas with relics of primary muscovite also occur. Late micas are derived from a phase melt enriched in F and Li.

Keywords

granitesaplite-pegmatite sillsfeldsparsmicascolumbite-tantalite