Isograds and P–T evolution in the eastern Lepontine Alps (Graubünden, Switzerland)

Reactions producing Al‐rich index minerals in the south‐eastern part of the Lepontine Dome (Central Alps, Switzerland) are investigated using mineral distribution maps, microstructural observations and equilibrium phase diagrams. The apparent staurolite mineral zone boundary corresponds to the paragonite breakdown reaction Pg + Grt + Qtz = Pl + Al₂O₃ + W. Equilibrium phase diagrams show that most natural metapelites do not contain staurolite or alumosilicates as long as univalent cations are predominantly accommodated in white mica. For a wide range of metapelitic compositions the paragonite breakdown releases sufficient Al for the formation of these minerals. Rare occurrences of staurolite and kyanite, north of the formerly mapped mineral zone boundaries, coexist with paragonite and are restricted to extremely Al‐rich bulk compositions. The stable branch of the kyanite‐forming paragonite breakdown reaction above 660 °C yields an additional mapable isograd. The second set of Al‐releasing reactions is biotite‐producing phengite breakdown. However, these reactions are less suitable to produce well defined reaction isograds in the field as they are more continuous and their progress is strongly dependent on bulk composition. Well developed fibrolite in metapelites does not appear until staurolite starts to breakdown. We conclude that amphibolite facies conditions in the study area were attained by decompression, without substantial heating at low pressures.