Petrogenesis of Anorthosite Complexes

Anorthosite complexes are large, plagioclase-rich intrusions that record fundamental processes of crustal differentiation and mantle–crust interaction. They typically form when high-aluminium basaltic magmas undergo extensive fractional crystallisation, leading to buoyant separation of plagioclase from denser mafic phases. In many cases parental magmas ascend through the lithosphere, experiencing polybaric crystallisation at depths spanning 5–40 km. During ascent, progressive decompression and cooling drive sequential crystallisation of olivine and pyroxene, followed by the accumulation of plagioclase crystals to form metre- to kilometre-scale anorthosite bodies. Compositional zoning in associated mafic cumulates and presence of high-aluminium orthopyroxene megacrysts testify to complex multi-stage growth histories. Thermodynamic and geochemical models suggest that parental melts may derive from partial melting of mantle sources contaminated by lower-crustal material at Moho levels. Subsequent isobaric and polybaric fractional crystallisation yields residual melts that crystallise as monzodiorites and oxide-rich gabbronorites, which often accompany the anorthosite core. Many massif-type anorthosites display stratigraphy defined by alternating plagioclase-rich and ferromagnesian cumulate layers, reflecting periodic replenishment, magma mixing and density-driven crystal sorting. Beyond their petrogenetic significance, these complexes host important Fe–Ti–V ore bodies and carry a record of Proterozoic to Archean tectonic settings, from rift-related to collisional regimes.

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Petrogenesis of Anorthosite Complexes publication trend

The graph below shows the total number of articles in petrogenesis of anorthosite complexes across all publications each year (not limited to Nature Index journals).

Technical terms

Anorthosite: A coarse-grained igneous rock composed predominantly of plagioclase feldspar, often forming large intrusive bodies.

Cumulate: An igneous rock layer formed by accumulation of early-crystallising minerals, typically denser phases that settle from a magma.

Polybaric crystallisation: Sequential crystallisation of magma under varying pressures during ascent through the crust.

Fractional crystallisation: The process by which crystals are removed from a cooling magma, changing the composition of the residual liquid.

Residual melt: The liquid remaining after removal of early-formed crystals during fractional crystallisation.

Monzodiorite: An intermediate intrusive rock rich in plagioclase and ferromagnesian minerals, often representing evolved melts in anorthosite systems.

References

1. Fe–Ti(–V) Oxide Deposits of the Kunene Anorthosite Complex (SW Angola): Mineralogy and Thermo-Oxybarometry. Minerals (2017).
2. High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites. Contributions to Mineralogy and Petrology (2019).
3. Equilibrium crystallization of massif-type anorthosite residual melts: a case study from the 1.64 Ga Ahvenisto complex, Southeastern Finland. Contributions to Mineralogy and Petrology (2020).
4. Thermodynamic constraints on the petrogenesis of massif-type anorthosites and their parental magmas. Lithos (2022).