THE TEMPORALITY OF ANORTHOSITES

The compositions, textures, associated rocks and tectonic environments of anorthosites allow categorization into six different types, some of which show distinct age-restrictions in terrestrial and planetary history. Primordial, Archean and Proterozoic anorthosites are distinctly different and are restricted in time and space. Those of layered mafic intrusions and ophiolites show no apparent time-restrictions. Primordial anorthosites form the bulk of the lunar crust and possibly that of Mercury, and probably formed in response to planetary accretion. No surviving crust of this nature has yet been found on Earth. Calcic (>An(80)) megacrystic anorthosites are restricted entirely to the Archean, where they are associated with mafic volcanic rocks of greenstone belts, and may have formed by accumulation of An-rich plagioclase from mafic to ultramafic magmas; high H2O content of parental melts may explain the calcic compositions, and the temporality might be linked to that of komatiitic magmatism. Massif-type anorthosite is the most abundant of terrestrial types, and occurs as small plutons to huge composite batholiths that are almost entirely restricted to the Proterozoic, although the vast majority were emplaced between similar to 1000 and similar to 1800 Ma. Compositions (An(50+10)), textures (lathy plagioclase) and settings (emplaced into continental rather than oceanic crust) effectively distinguish Proterozoic from Archean anorthosites. Their time restriction may be related to unique Proterozoic supercontinental configurations. Incomplete preservation or insufficient depth of erosion are unlikely explanations for the temporality of Archean and Proterozoic anorthosite types, and some degree of non-unformitarianism is therefore implied.