Trace elements, particularly rare earths, determined in Archean komatiitic and tholeiitic rocks of Munro Township, Ontario, appear mainly to reflect the processes that produced the lavas, rather than the metamorphic processes. Pyroxenitic to basaltic komatiites may have originated by fractional crystallization of peridotitic komatiite liquid or by partial melting of plagioclase peridotite in the mantle. Peridotitic komatiite magmas may be partial melts of a residual mantle assemblage that was depleted in largeion lithophile (LIL) elements, possibly through earlier extraction of liquids. Picritic tholeiites may have originated from melting of garnet peridotite.
A multistage melting model may relate both tholeiites and komatiites to a common source, perhaps in a rising mantle diapir. In this model tholeiites would be the first-formed liquids extracted whereas residual mantle might be further melted to produce komatiitic liquids.