Mg‐suite noritic rocks can be divided into two groups, the Mg‐gabbronorites and the Mg‐norites. The rocks of these 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. The two groups probably crystallized from different types of parent magmas. Two hypotheses are offered for the differences in composition of the parent magmas. One hypothesis ascribes the differences to compositional heterogeneity of the mantle source areas. The other hypothesis ascribes the differences to variations in extent of partial melting of the mantle source regions and variations in extent of assimilation of the anorthosite and the highly differentiated residual liquid that were produced during the primordial lunar differentiation.
Clasts of an unusual type of lunar highlands igneous rock, alkali gabbronorite, have been found in Apollo 16 breccia 67975. The alkali gabbronorites form two distinct subgroups, magnesian and ferroan. Modes and bulk compositions are highly varied. The magnesian alkali gabbronorites are composed of bytownitic plagioclase (Or2–5An82–89), hypersthene (Wo3–5En49–62), augite (Wo39–42En36–44), a silica mineral, and trace Ba‐rich K‐feldspar. The ferroan alkali gabbronorites are composed of ternary plagioclase (Or11–22An65–74), pigeonite (Wo6–9En35–47), augite (Wo38–40En29–35), Ba‐rich K‐feldspar, and a silica mineral. Trace minerals in both subgroups are apatite, REE‐rich whitlockite, and zircon. The magnesian and ferroan alkali gabbronorites appear to have formed by progressive differentiation of the same, or closely related, parent magmas; the compositional data indicate that these magmas were REE‐rich. The ternary plagioclase is probably a high‐temperature metastable phase formed during crystallization. In composition and mineralogy, the 67975 alkali gabbronorites show many similarities to Apollo 12 and 14 alkali norites, alkali gabbronorites, and alkali anorthosites, and all these rocks together constitute a distinctive alkali suite. In addition, the alkali gabbronorites show some similarities to KREEP basalts, Mg‐norites, and some felsites. These data suggest genetic links between some or all of these types of pristine rocks. Two types of relationships are possible. The first is that alkali‐suite rocks crystallized in plutons of KREEP basalt magma, and KREEP basalts are their extrusive equivalents. The second is that the alkali‐suite rocks and some felsites all crystallized in plutons of Mg‐norite parent magmas, and KREEP basalt magmas formed by remelting of these plutons. Additional studies are needed to resolve which of these hypotheses is correct.
Dimict breccia 61015 consists dominantly of granulated anorthosite and impact melt rock of VHA (very‐high‐alumina)‐basalt composition. The anorthosite is a typical Apollo 16 ferroan anorthosite, except that it is slightly more augite‐rich than most other ferroan anorthosites. The breccia probably formed in the floor of an expanding crater cavity by the following process: Impact melt generated within the cavity was injected into underlying anorthositic bedrock and solidified rapidly, and the anorthosite and melt rock were then fragmented by deformation in the later stages of the impact. All the Apollo 16 dimict breccias (except perhaps 61016) appear to have formed in the same impact. The source crater cannot yet be identified, but it may be an intermediate‐sized (50–150 km diameter) crater within a few hundred kilometers of the Apollo 16 site, in the direction of the Imbrium basin, of either pre‐Nectarian or post‐Nectarian/pre‐Imbrian age. The breccia has been affected by at least one, and perhaps as many as four, recent small impacts (the most recent was probably the South Ray impact). The recent impact(s) produced internal shock‐induced melting of the breccia and injected and coated the rock with externally derived impact melts. The coating glass is heterogeneous. Its average composition is similar to the compositions of other Apollo 16 coating glasses, except that its TiO2 content is lower. The vein glass is unusually mafic and has very low TiO2 for its Al2O3 content. The TiO2 contents of these glasses establish that they cannot be melted soils; their compositions require a mafic, low‐TiO2 protolith component that has not yet been identified among the large rock samples, perhaps a very mafic member of the ferroan‐anorthosite suite.
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