Moonrocks - Classification - Highlands Lithologies

Highlands Lithologies

Mineral composition of Highland rocks
Plagioclase Pyroxene Olivine Ilmenite
Anorthosite 90% 5% 5% 0%
Norite 60% 35% 5% 0%
Troctolite 60% 5% 35% 0%
Mineral composition of mare basalts
Plagioclase Pyroxene Olivine Ilmenite
High titanium content 30% 54% 3% 18%
Low titanium content 30% 60% 5% 5%
Very low titanium content 35% 55% 8% 2%
Common lunar minerals
Mineral Elements Lunar rock appearance
Plagioclase feldspar Calcium (Ca)
Aluminium (Al)
Silicon (Si)
Oxygen (O)
White to transparent gray; usually as elongated grains.
Pyroxene Iron (Fe),
Magnesium (Mg)
Calcium (Ca)
Silicon (Si)
Oxygen (O)
Maroon to black; the grains appear more elongated in the maria and more square in the highlands.
Olivine Iron (Fe)
Magnesium (Mg)
Silicon (Si)
Oxygen (O)
Greenish color; generally, it appears in a rounded shape.
Ilmenite Iron (Fe),
Titanium (Ti)
Oxygen (O)
Black, elongated square crystals.

Primary igneous rocks in the lunar highlands compose three distinct groups: the ferroan anorthosite suite, the magnesian suite, and the alkali suite.

Lunar breccias, formed largely by the immense basin-forming impacts, are dominantly composed of highland lithologies because most mare basalts post-date basin formation (and largely fill these impact basins).

The ferroan anorthosite suite consists almost exclusively of the rock anorthosite (>90% calcic plagioclase) with less common anorthositic gabbro (70-80% calcic plagioclase, with minor pyroxene). The ferroan anorthosite suite is the most common group in the highlands, and is inferred to represent plagioclase flotation cumulates of the lunar magma ocean, with interstitial mafic phases formed from trapped interstitial melt or rafted upwards with the more abundant plagioclase framework. The plagioclase is extremely calcic by terrestrial standards, with molar anorthite contents of 94-96% (An94-96). This reflects the extreme depletion of the bulk moon in alkalis (Na, K) as well as water and other volatile elements. In contrast, the mafic minerals in this suite have low Mg/Fe ratios that are inconsistent with calcic plagioclase compositions. Ferroan anorthosites have been dated using the internal isochron method at "circa" 4.4 Ga.

The magnesian suite (or "mg suite") consists of dunites (>90% olivine), troctolites (olivine-plagioclase), and gabbros (plagioclase-pyroxene) with relatively high Mg/Fe ratios in the mafic minerals and a range of plagioclase compositions that are still generally calcic (An86-93). These rocks represent later intrusions into the highlands crust (ferroan anorthosite) at round 4.3-4.1 Ga. An interesting aspect of this suite is that analysis of the trace element content of plagioclase and pyroxene require equilibrium with a KREEP-rich magma, despite the refractory major element contents.

The alkali suite is so-called because of its high alkali content -- for moon rocks. The alkali suite consists of alkali anorthosites with relatively sodic plagioclase (An70-85), norites (plagioclasse-orthopyroxene), and gabbronorites (plagioclase-clinopyroxene-orthopyroxene) with similar plagioclase compositions and mafic minerals more iron-rich than the magnesian suite. The trace element contents of these minerals also indicates a KREEP-rich parent magma. The alkali suite spans an age range similar to the magnesian suite. Lunar granites are relatively rare rocks that include diorites, monzodiorites, and granophyres. They consist of quartz, plagioclase, orthoclase or alkali feldspar, rare mafics (pyroxene), and rare zircon. The alkali feldspar may have unusual compositions unlike any terrestrial feldspar, and they are often Ba-rich. These rocks apparently form by the extreme fractional crystallization of magnesian suite or alkali suite magmas, although liquid immiscibility may also play a role. U-Pb date of zircons from these rocks and from lunar soils have ages of 4.1-4.4 Ga, more or less the same as the magnesian suite and alkali suite rocks. In the 1960s, NASA researcher John A. O'Keefe and others linked lunar granites with tektites found on Earth although many researchers refuted these claims. According to one study, a portion of lunar sample 12013 has a chemistry that closely resembles javanite tektites found on Earth.

Lunar breccias range from glassy vitrophyre melt rocks, to glass-rich breccia, to regolith breccias. The vitrophyres are dominantly glassy rocks that represent impact melt sheets that fill large impact structures. They contain few clasts of the target lithology, which is largely melted by the impact. Glassy breccias form from impact melt that exit the crater and entrain large volumes of crushed (but not melted) ejecta. It may contain abundant clasts that reflect the range of lithologies in the target region, sitting in a matrix of mineral fragments plus glass that welds it all together. Some of the clasts in these breccias are pieces of older breccias, documenting a repeated history of impact brecciation, cooling, and impact. Regolith breccias resemble the glassy breccias but have little or no glass (melt) to weld them together. As noted above, the basin-forming impacts responsible for these breccias pre-date almost all mare basalt volcanism, so clasts of mare basalt are very rare. When found, these clasts represent the earliest phase of mare basalt volcanism preserved.

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