Petrogenesis of Tin-bearing Granites from Ervedosa, Northern Portugal: The Importance of Magmatic Processes

Abstract

Three Hercynian highly peraluminous tin-bearing granites define a sequence ranging from muscovite-biotite granite to muscovite granite. Tin-bearing quartz veins are genetically related to this sequence. Variation diagrams of most major and trace elements of granites, biotite and muscovite show fractionation trends. Least squares analysis of major elements and modelling of trace elements indicate that the muscovite-biotite granite M2 and the muscovite granite M3 were derived from the slightly porphyritic muscovite-biotite granite magma M1 by fractional crystallization of plagioclase, K-feldspar, biotite and quartz. The granite magma M1 was originated by partial fusion of peraluminous metasedimentary crustal materials. The magmatic fractionation was responsible for the increase in Sn contents of granites and their micas. Biotite has higher Sn content than coexisting muscovite. However, muscovite retains a higher percentage of the total granite Sn content, up to 99 % of the uscovite granite. The very rare magmatic cassiterite present in muscovite granite M3 confirms the tin enrichment of magma. In the sequence, the melt temperature decreases from 765 to 735 C, PH2O decreases from 4 to 3 kb, and the F content in melt increases. Feldspars reequilibrated at 567-329 °C.