Genesis of massif-type anorthosites — the role of high-grade metamorphism

The genesis of massif-type anorthosites in general is discussed on the basis of data obtained on the Capivarita massif, a labradorite-type anorthosite from Southern Brazil. Massif anorthosites are thought to have originated by fractional crystallization of magmas of suitable compositions (essentially high-alumina basalt and andesite melts) that underwent extremely slow cooling under high-grade metamorphic conditions. Plagioclase is the dominant liquidus phase in the melt only over a very restricted temperature interval (10–20 °C) in which water pressure (PH₂O<P total) is maintained more or less constant at a critical value. Plagioclase accumulation by gravitational sinking is operative only under a set of critical conditions that are statistically unlikely to occur or to be maintained for any length of time. Hence, anorthosites would be relatively rare. The possible influence of the total pressure on anorthosite composition is discussed.At present, Chief Research Worker of the Brazilian CNPq.