The temporality of anorthosites and insights into Earth evolution

Thursday, 18 December 2014: 10:20 AM
Lewis D Ashwal, University of the Witwatersrand, School of Geosciences, Johannsburg, South Africa
Several types of anorthosite can be recognized, some of which show distinct age restrictions in Earth 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 have been provisionally detected on Mars; these probably formed in response to planetary accretion. No surviving crust of this nature has yet been found on Earth. Calcic (>An80) megacrystic anorthosites are restricted to the Archean, are associated with mafic volcanics 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, for which many tectonic settings, including subduction, have been proposed. Massif-type anorthosite is the most abundant of terrestrial types, and occurs as small plutons to huge composite batholiths that are entirely restricted to the Proterozoic. Compositions (An50 ± 10 vs. >An80), textures (lathy vs. equant plagioclase) and size (massifs to ~15,000 km2 vs. Archean bodies to ~560 km2) effectively distinguish Proterozoic from Archean anorthosites. An arc environment for massifs is suggested by the long, linear belts of coalescent anorthositic plutons, some of which have been emplaced into continental crust over extensive time periods (>100 m.y.). Magma derivation was from depleted mantle (not mafic lower crust), although there is evidence for substantial crustal contamination. Plagioclase-rich mushes formed by flotation in deep crustal chambers, and ascended diapirically to the mid-crust. Massif anorthosites have not yet been found in Phanerozoic continental arcs, although anorthositic layers are present as parts of layered gabbros in some deep crustal sections (e.g. Argentina). It is not clear if different subduction styles alone can explain the differences between Archean, Proterozoic and Phanerozoic anorthosites. Their temporality might be caused by any combination of secular cooling of the Earth, secular variation in H2O content of arc magmas and insufficient depths of erosion of young arc terranes.