GENESIS OF HIGH MG-NUMBER ANDESITES AND THE CONTINENTAL-CRUST

The continental crust has an andesitic composition with high Mg/(Mg + Fe) and Ni contents which may be too high to have formed by differentiation of basaltic magmas. Instead, mantle-derived, high Mg# andesites (HMA) may form a substantial component of the crust. HMA maybe produced by partial melting of of previously depleted, subsequently metasomatised mantle peridotite. However, they are more likely produced by reaction between ascending melts and mantle peridotite. HMA are less common than basalts among lavas in modern island area, but may have been more common in the past, may be produced in specific environments (such as ''ridge subduction''), may be more common among plutonic rocks in the lower and middle crust than among lavas at the surface, and may be selectively preserved during later erosion and subduction processes.