Mesozoic decratonization of the North China block

Cratons are old, stable parts of the continental crust that have survived at least since Proterozoic rime and have not undergone strong magmatism or tectonism since their stabilization. Traditionally, the growth of continental crust in the North China craton is considered to have been complete by Paleoproterozoic time, Phanerozoic events being largely restricted to surrounding orogenic belts. However, the eastern part of the North China craton contains large volumes of Mesozoic igneous rocks, with widespread metamorphic core complexes and pull-apart basins. Hf isotope compositions of magmatic zircon grains from igneous rocks in the Liaodong Peninsula indicate that widespread late Mesozoic granitoids formed by partial melting of ancient crust, but with significant input of a mantle component via magma mixing and crustal assimilation. This magmatism has resulted from removal and modification of lithospheric mantle, accompanied by asthenospheric upwelling. The Hf isotopic signatures thus record the addition of juvenile crust beneath the eastern part of the North China craton, which appears related to major extension and possibly slab rollback of the Pacific plate. Whatever the mechanism, it is apparent that since ca. 200 Ma, ancient lithosphere beneath the eastern North China craton has been progressively reactivated and replaced, resulting in 'decratonization.' Tertiary to Holocene volcanism in the area and major subsidence around the Gulf of Bohai suggest that the effects of the process are continuing. Similar processes probably operated in the geological past, leading to significant modification of continental crust and requiring reconsideration of mechanisms for continental breakup and dispersal and of continental growth rates. Keywords: decratonization, Phanerozoic crustal growth, North China craton, zircon, Hf isotope, magmatism.