Mg-metasomatism of metagranitoids from the Alps: genesis and possible tectonic scenarios

Mg-metasomatic rocks (e.g. whiteschists, leucophyllites) de- rived from post-Variscan granitoids are common in the Alps. Previously reported field, petrological, geochemical and fluid inclusion data are combined to trace the genetic processes and the associated tectonic scenarios. Although the heterogeneous data, many common features can be recognised in all of the continental Mg-metasomatic rocks, indicating that the genetic process is likely common in the entire range of the Alps. This process assumes highly channelised fluids – derived from ultramafic rocks previously interacting with seawater – that infiltrated the continental crust along strain zones and pro- duced chromatographic fractionation of major and trace elements. Three tectonic scenarios, involving distinct mantle sources, are proposed: rift-related ocean-continent transition, continental subduction and continent–continent collision. All these data suggest that the Mg-metasomatism was diachronous and occurred at different structural levels during the Alpine history.