Abstract-New data are reported from five previously unanalyzed Apollo 12 mare basalts that are incorporated into an evaluation of previous petrogenetic modelsand classification schemes for these basalts. This paper proposes a classification for Apollo 12 mare basalts on the basis of whole-rock Mg# [molar 100·(MgI(Mg+Fe»] and RbiSr ratio (analyzed by isotope dilution), whereby the ilmenite,.olivine, and pigeonite basalt groups are readily distinguished from each other. Scrutiny ofthe Apollo 12 feldspathic "suite" demonstrates thl¢ two of the three basalts previously assigned to this group (12031, 12038, 12072) can be reclassified: 12031 is a plagioclase-rich pigeonite basah (Nyquist et al., 1979); and 12072 is an olivine basalt, Only basah 12038 stands out as a unique sample (Nyquist et al., 1981) to the Apollo 12 site, but whether this represents a single sample from another flow at the Apollo 12 site or is exotic to this site is equivocal.The question of whether the olivine and pigeonite basalt suites are co-magmatic is addressed by incompatible trace-elernent chemistry: the trends defined by these two suites when Co/Sm andSmIEu ratios are plotted against RblSr ratio demonstrate that these two basaltic types cannot be co-magmatic. Crystal fractionation/accumulation paths have been calculated and show that neither the pigeonite, olivine, or ilmenite basalts are related by this process. Each suite requires a distinct andseparate source region. This study also examines sample heterogeneity and the degree to which whole-rock analyses are representative, which is critical when petrogenetic interpretation is undertaken. Sample heterogeneity has been investigated petrographically (inhomogeneous mineral distribution) with consideration of duplicate analyses, and whether a specific sample (using average data) plots consistently upon a fractionation trend when a number of different compositional parameters are considered. Using these criteria, four basalts have been identified where reported analyses are not representative ofthe whole-rock composition: 12005, an ilmenite basalt; 12006 and 12036, olivine basalts; and 12031 previously classified as a feldspathic basalt, but reclassified as part ofthepigeonite suite (Nyquist et al., 1979).

Andesitic and dacitic lavas erupted from Quaternary stratovolcanoes in the Central Volcanic Zone (CVZ) of the Andes have major and trace element compositions similar to proposed melts of subducted mid-ocean ridge basalt and Na-rich trondjhemites, tonalites, and granodiorites (TTG) in Early Archean high-grade gneiss terrains. Suites of lavas from individual volcanoes have average La/Yb of 25-45, Sr/Y of 30-65, Yb contents of 1.5-1.1 ppm, and Na O contents > 3.8 wt %, with lavas erupted on the volcanic front displaying the most affinity to the Early Archean suites. Because slab-melting is not involved in the derivation of the CVZ magmas, these data indicate that rocks with Early Archean TTG compositions can also be produced through intracrustal differentiation processes. Isotopic, major element, and REE data from Volcan Ollagiie are consistent with a model incorporating contamination of mantle-derived basaltic magmas in garnet-bearing deep continental crust to produce basaltic andesite magma, followed by fractionation of a plagioclase-dominated mineral assemblage in shallow crustal magma chambers to produce andesitic and dacitic magma with compositional features similar to Na-rich TTG. Slight offset between parental basaltic andesite and andesite samples on diagrams illustrating variation of REE suggest that the shallow crustal magma chambers are density stratified with basaltic andesite at the base overlain by zones of andesitic and dacitic magma. Furthermore, strongly increasing La/Yb and decreasing '43Nd/'44Nd ratios with decreasing Yb contents for basaltic andesite samples indicate that the deep continental crust beneath Ollagiie is probably more silicic and enriched in incompatible trace elements relative to crust beneath volcanoes located farther west along the present arc front of the central Andes. Based on these results, it is suggested that contamination of mantle-derived basaltic magmas in garnet-bearing mafic continental crust is an alternative mechanism to slab-melting to produce the Early Archean TTG suites and younger rocks with similar compositions. This model and the data summarized here may provide support for models advocating relatively early growth of the earth's continental crust.

Abstract— The petrogenesis of Apollo 12 mare basalts has been examined with emphasis on trace‐element ratios and abundances. Vitrophyric basalts were used as parental compositions for the modelling, and proportions of fractionating phases were determined using the MAGFOX program of Longhi (1991). Crystal fractionation processes within crustal and sub‐crustal magma chambers are evaluated as a function of pressure. Knowledge of the fractionating phases allows trace‐element variations to be considered as either source related or as a product of post‐magma‐generation processes. For the ilmenite and olivine basalts, trace‐element variations are inherited from the source, but the pigeonite basalt data have been interpreted with open‐system evolution processes through crustal assimilation. Three groups of basalts have been examined: (1) Pigeonite basalts — produced by the assimilation of lunar crustal material by a parental melt (up to 3% assimilation and 10% crystal fractionation, with an “r” value of 0.3). (2) Ilmenite basalts — produced by variable degrees of partial melting (4–8%) of a source of olivine, pigeonite, augite, and plagioclase, brought together by overturn of the Lunar Magma Ocean (LMO) cumulate pile. After generation, which did not exhaust any of the minerals in the source, these melts experienced closed‐system crystal fractionation/accumulation. (3) Olivine basalts — produced by variable degrees of partial melting (5–10%) of a source of olivine, pigeonite, and augite. After generation, again without exhausting any of the minerals in the source, these melts evolved through crystal accumulation. The evolved liquid counterparts of these cumulates have not been sampled. The source compositions for the ilmenite and olivine basalts were calculated by assuming that the vitrophyric compositions were primary and the magmas were produced by non‐modal batch melting. Although the magnitude is unclear, evaluation of these source regions indicates that both be composed of early‐ and late‐stage Lunar Magma Ocean (LMO) cumulates, requiring an overturn of the cumulate pile.