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It is proposed that zoned ultramafic complexes of the Alaskan type originate by fractional crystallization and flow differentiation of basaltic magma in the feeder pipes of volcanoes.

The parent magma of the complexes may be an olivine-rich tholeiitic basalt. Crystallization under hydrous conditions in the pressure range 2 to 11 kbars can produce the mineral assemblages of most zoned complexes. Segregation of crystals along the axis of the conduit by flow differentiation as the magma rises would produce the observed mineralogical and chemical zoning.

Separation of varying proportions of ultramafic materials from the parent magma could produce high-alumina tholeiitic basalt and andesite. Volcanic rocks of this type are spatially and temporally associated with ultramafic complexes in several areas.

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