Archean Megacrystic Anorthosites

Anorthosite has been recognized as a component of Archean terranes since at least the 1930s. Interestingly, the early studies of Archean anorthosite were made on occurrences at some of the highest metamorphic grades, for example in Madagascar, India, South Africa, and West Greenland (e.g., Lacroix 1939; Sohnge 1946). A new, third type of anorthosite, separate from those which occur as Proterozoic “massifs” (Chap. 3) or as components of mafic layered intrusions (Chap. 4), was suggested by Boulanger (1957, 1959) based on occurrences near the village of Sakeny, Madagascar. These and similar occurrences of highly metamorphosed calcic anorthosite were referred to as Sakeny-type anorthosites, and were thought to have formed by high grade metamorphism of calcareous sediments. It was not until the discovery of associated layered chromitites (e.g., Subramaniam 1956a; Ghisler 1966) that a convincing case could be made that these anorthosites represented components of layered igneous complexes. The pioneering work of B.F. Windley and colleagues in the 1960s and 1970s, and J.S. Myers in the 1970s and 1980s, particularly on the Fiskenæsset complex of West Greenland set the stage for modern investigations. More recent work by W.C. Phinney and colleagues has focused on the lower grade Archean anorthosite occurrences, particularly in the Canadian Shield.