High-pressure fractionation in rift-related basaltic magmatism: Faeroe plateau basalts

The lower 3000 m basalt stratigraphy on the Faeroe Islands shows strong positive correlation between degree of fractionation and Zr/Y ratio. This is ascribed to the presence of garnet in the cumulus assemblage during fractionation at the base of the continental crust or in the upper lithospheric mantle at 40-50 km depth. Later basaltic magmas fractionated at low pressures, which is a typical shift in fractionation regime among continental flood basalts. Seismic velocity (V _p ) for the garnet-bearing high-pressure fractionates is in excess of 7 km/s. This is comparable to velocities observed in high-V _p bodies in the basal crust of Precambrian shields. High-pressure fractional crystallization of basaltic magmas may thus be an important process in generating the chemical complexity of continental flood basalts and has implications for crustal underplating. -Author