Preservation of Archean mafic lower continental crust worldwide

Abstract

Mafic granulite xenoliths found in Archean cratons have generally been inferred to form from post-Archean basaltic underplating, implying that the Archean mafic lower crust was removed perhaps via delamination. Here based on an integrated study of granulite terrain, granulite xenoliths and lower crustal-derived granitoids, we make a compelling case that most of the Tertiary alkali basalt-entrained mafic granulite xenoliths from the northern North China craton are restites left after partial melting of the late Archean mafic lower crust to produce the Mesozoic granitoids. This runs counter to archetypical Mesozoic basaltic underplating as commonly believed. We further show that the Archean mafic lower crust persists on a cratonic scale in the North China craton presently. Re-examination of mafic granulite xenoliths from many other cratons that were considered to be products of Proterozoic underplating shows that most of them have similar genetic mechanism to the present case, suggesting that they may form in the Archean. This attests to preservation of the Archean mafic lower crust globally, contrary to the widely held concept that much of the Archean mafic lower crust has delaminated. We predict preservation of such mafic lower crust beneath all Archean cratons. It suggests that the quantity of Archean rocks in the lower crust may be much greater than that exposed.