Mafic crustal xenoliths and the origin of the lower continental crust

The La/Nb ratio (<1.4) and the Ni concentration (>30 ppm at a Mg number of 40) in oceanic plateau basalts and MORB provide a useful index to distinguish them from arc basalts. MORB can generally be distinguished from plateau basalts by their lower Nb contents (<3 ppm) and lower Nb/Y ratios (<0.1). The La/Nb ratio and Ni contents of mafic lower crustal xenoliths indicate that at most xenolith sites, both arc and plume-related components occur in the lower crust. Considered as a whole, mafic xenolith data suggest that about one third of the post-Archean lower continental crust is composed of mafic rocks from mantle plume sources (accreted oceanic plateaus or mafic underplate), and the remainder come chiefly from arc sources. Because both crustal and host magma contamination should raise La/Nb ratios in xenoliths, the value of one third is a minimum for the plume component in the lower crust.