Abstract
Publisher Summary Anorthosite forms a minor but prominent component of Archean crust and can be found associated with mafic intrusives and extrusives of many (but not all) greenstone belts. These anorthosites are characterized by megacrysts of equidimensional calcic plagioclase (usually Ansms), and form extensive sheet-like bodies, mostly emplaced at shallow depths into basaltic volcanic rocks. Original thicknesses of the anorthositic sheets are difficult to determine because of deformation. Archean anorthosites were derived from fractionated basaltic magmas (Fe-rich tholeiites), and related basaltic rocks with calcic plagioclase megacrysts also form dikes and flows. The major anorthosite intrusions comprise layered components of anorthosite, leucogabbro, gabbro, melanogabbro, and ultramafic rocks. They contain rocks, textures, and structures similar to those of younger layered basic intrusions, but are generally distinguished by the abundance of anorthosite and leucogabbro over gabbro and ultramafic rocks, and the equidimensional megacrysts of calcic plagioclase.
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