Subdivision of the Mg-suite noritic rocks into Mg-gabbronorites and Mg-norites.

The rocks of these two groups differ in ratios of high-Ca pyroxene to total pyroxene, compositions of pyroxene and plagioclase, assemblages of Ti-, Nb- and Zr-bearing minerals, compositions of chrome spinel, bulk-rock Ti/Sm and Sc/Sm, and measured ages. Two hypotheses are offered for differences in composition of the parent magmas: 1) compositional heterogeneity of the mantle source areas; 2) variations in extent of partial melting of mantle source regions and variations in extent of assimilation of the anorthosite and highly differentiated residual liquid that were produced during the primordial lunar differentiation.-P.Br.