Petrologists are still trying to solve the 'anorthosite problem', which was first identified by N.L. Bowen in 1917 and revolves around the uncertainty surrounding the petrogenesis, tectonic setting, emplacement, and parental magma compositions of anorthosites. Anorthosites are volumetrically minor to significant, occur worldwide and have implications for upper mantle depletion, oceanic and continental crust growth and evolution, the longevity of plate tectonic processes on Earth, and magma chamber processes. Anorthosites have formed throughout Earth history from 3950 Ma to the Present. Anorthosites are far more common than previously thought as there are at least 2,126 known preserved anorthosite occurrences worldwide. Anorthosites are closely associated with gabbros and leucogabbros and spatially associated with pillowbearing mafic volcanic rocks in ophiolites and layered intrusions, except for Proterozoic anorthosites that are closely associated with intermediate to felsic intrusives. Archean anorthosites often contain near-spherical calcic plagioclase (An70-100) megacrysts up to 45 cm in diameter and magmatic amphibole, Proterozoic anorthosites have elongate (<= 1 m) andesine (An30-50) megacrysts, and Phanerozoic anorthosites contain calcic plagioclase (An70-100). The plagioclase anorthite contents of anorthosites vary throughout Earth history, troughing in the Proterozoic. This corroborates the whole-rock geochemistry of anorthosites, which also varies throughout Earth history and indicates that crustal contamination played a significant role in the petrogenesis of Proterozoic anorthosites. Relatively high mantle heat flow and partial melting and the thermal blanketing of Proterozoic supercontinents led to the generation of large anorthosite massifs. Archean, Proterozoic and Phanerozoic anorthosites crystallised from tholeiitic or high-Al tholeiitic parental magmas that were derived from depleted mantle sources and significantly assimilated continental crust in the Proterozoic. Archean anorthosites crystallised from hydrous Caand Al-rich tholeiitic parental magmas derived by partial melting of hydrated sub-arc depleted mantle sources. Anorthosites have predominantly formed in subduction-related settings throughout Earth history, indicating that plate tectonic processes have operated since the beginning of the Eoarchean. The proportion of subductionrelated anorthosites significantly declined in the Proterozoic following supercontinent amalgamation. Calcic megacrystic anorthosites and massif-type anorthosites are largely restricted to the Archean and Proterozoic, respectively, yet they occur throughout Earth history. The petrogenesis of anorthosites reflects the secular evolution of the Earth.
