Are Bushveld U-type parent magmas boninites or contaminated komatiites?

Three distinct categories of magmas — Bushveld U-type parent magmas, boninites, and siliceous high magnesium basalts from Archaean greenstone belts —share the distictive geochemical characteristics of high MgO (9%–19%), low TiO₂(less than 1%) and high SiO₂(greater than 52%). Boninites are generally thought to form by hydrous melting of metasomatized, previously depleted upper mantle, while siliceous high magnesium basalts (SHMB) in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites. Both these mechanisms can apparently give rise to similar end products, and both mechanisms have been proposed for the petrogenesis of Bushveld U-type magmas.A detailed comparison of the three magma types, using data drawn from the literature, shows a broad area of overlap in major elements and most trace elements. U-type magmas are generally intermediate in composition between SHMB and boninites. U-type magmas differ significantly from boninites, and are more similar to SHMB, in three important respects: their relatively high abundances of rare earth elements and degree of light rare earth enrichment; higher FeO/MgO ratio for a given MgO content; and Sm/Nd isotopic systematics indicative of crustal contamination. BU magmas are therefore more likely to be extreme examples of contaminated komatiitic parents than primary boninitic mantle melts. The striking similarity in major element chemistry of the three groups may be due to the near-coincidence in compositional space of the mediumpressure, hydrous olivine-orthopyroxene phase boundary, which controls the composition of boninites, with the lowpressure anhydrous phase boundary which controls differentiated SHMB and U-type magmas.