Origin and Formation of Tourmaline-rich Cordierite-bearing Metapelitic Rocks from Alpe Sponda, Central Alps (Switzerland)

Tourmaline is known as a forensic mineral that can be used to reconstruct the history of its host rock. Metamorphic rocks are important hosts of tourmaline, where it is stable over a wide P–T range. At Alpe Sponda, tourmaline occurs as an accessory mineral in paragonite-free kyanite–staurolite-bearing metapelitic schists, which at Pizzo Forno contain several lenses of tourmaline-rich, cordierite–garnet–kyanite–paragonite–biotite schists. These lenses are mineralogically distinct, as they lack both quartz and staurolite, and they contain large amounts of tourmaline (up to ∼20–25 vol. %). The samples from these lenses are rich in Li and B and exhibit an unusual bulk-rock composition characterized by low Si, but high Al, Mg and Na contents. All samples are enriched in light rare earth elements (REE) relative to the heavy REE and have a minor negative Ce anomaly (average Ce/Ce* = 0·91 ± 0·02), as well as a pronounced negative Eu anomaly (average Eu/Eu* = 0·58 ± 0·03). Tourmaline is dravite and/or ‘oxy-dravite’ with a tendency to povondraite, showing strong colour and chemical zoning. Garnet is also chemically zoned, and both tourmaline and garnet grew during prograde metamorphism. Cordierite has elevated Na and Be contents (Na–Be cordierite), and Li is mainly partitioned into this mineral. Based on the REE a development of tourmaline owing to hydrothermal processes such as metasomatism can be excluded. This is consistent with field relationships, which do not provide evidence for major influx of post-metamorphic fluids or partial melting. The layer from which the samples were collected can be interpreted as an evaporitic layer, which was probably deposited within a clay-rich environment in an intramontane basin. This conclusion is supported by tectonic discrimination diagrams, in which the Alpe Sponda samples plot in the field of an active continental margin. Mineral equilibrium modelling based on the original bulk-rock compositions yielded peak metamorphic conditions of amphibolite-facies grade (T = 590–660°C and P = 6·4–7·6 kbar), which is consistent with the P–T conditions estimated previously for this part of the Central Alps.