Discuss the origin of anorthosites.

Magmatic Origin Theories

The most widely accepted theories for anorthosite formation involve magmatic processes. These can be broadly categorized into fractional crystallization and magma accumulation.

Fractional Crystallization

This theory proposes that anorthosites form as a result of the preferential crystallization of plagioclase feldspar from a large, slowly cooling magma chamber. As the magma cools, plagioclase, having a high crystallization temperature, begins to crystallize and settle towards the bottom of the chamber due to its density. This process, repeated over time, leads to the accumulation of a plagioclase-rich residue – an anorthosite. The remaining liquid, depleted in plagioclase, would evolve towards more mafic compositions.

• Evidence: The compositional layering observed in some anorthosite massifs supports this theory. The presence of cumulate textures, where crystals are aligned due to flow and settling, is also indicative of fractional crystallization.
• Limitations: Explaining the extremely high plagioclase content (often >90%) solely through fractional crystallization is challenging, as other minerals would typically crystallize alongside plagioclase.

Magma Accumulation and Convection

This model suggests that large magma chambers develop beneath the crust, and convection currents within the chamber facilitate the concentration of plagioclase crystals in specific regions. The convection cells can create localized areas of plagioclase accumulation, eventually forming anorthosites. This is often linked to the idea of a ‘mush zone’ – a partially crystallized magma body.

• Evidence: Geophysical studies suggest the presence of large magma chambers beneath some anorthosite complexes.
• Limitations: The exact mechanisms driving the convection and the efficiency of plagioclase concentration are still debated.

Impact Origin Theories

The abundance of anorthosites on the Moon led to the development of impact origin theories, particularly in the context of the lunar highlands.

Lunar Magma Ocean Impact Excavation

This theory proposes that the early Moon experienced a period of intense bombardment, leading to the excavation of deep crustal material. The lunar magma ocean, a global molten layer, would have differentiated with plagioclase floating to the surface and forming the anorthositic crust. Subsequent impacts excavated this anorthositic crust, creating the lunar highlands.

• Evidence: The high albedo (reflectivity) and compositional homogeneity of the lunar highlands strongly support this theory. The presence of impact craters superimposed on the anorthositic crust further reinforces the idea of excavation.
• Limitations: Applying this model directly to terrestrial anorthosites is difficult, as Earth's crustal conditions were significantly different.

Metamorphic Origin Theories

While less common, metamorphic processes can also contribute to anorthosite formation.

Anatexis and Migmatization

This involves the partial melting of pre-existing crustal rocks, leading to the segregation of plagioclase-rich melts. These melts can then accumulate and crystallize to form anorthosites.

• Evidence: Some anorthosites exhibit evidence of partial melting and deformation.
• Limitations: This process typically produces smaller, localized anorthosite bodies rather than the large massifs observed in some regions.

Terrestrial vs. Lunar Anorthosites

While the fundamental processes may be similar, there are key differences between terrestrial and lunar anorthosites. Lunar anorthosites are generally more pristine and less altered than their terrestrial counterparts. Terrestrial anorthosites often show evidence of post-magmatic alteration and interaction with other rock types. The Bushveld Complex in South Africa and the Stillwater Complex in Montana are prime examples of terrestrial layered intrusions containing significant anorthositic components.