In some igneous rocks, zircon crystals are seen in circular shape that is probably because of the absorption of edges of the crystals by the primary magma
Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas
, may have produced the hydrated magnesium-poor basalts necessary to provide a source of the tonalite-trondhjemite-granodiorite complexes.
Variations in magma supply imply variations in the average degree of melting in the mantle, which affects the composition of primary magmas
coming out of the mantle and therefore the average composition of the oceanic crust.