Accretion and reworking beneath the North China Craton

How has the Earth's continental lithosphere evolved? Most of our knowledge is derived from surface exposures, but xenoliths carried in volcanic rocks can be an important source of information. The North China Craton (NCC) is one of the oldest in the world and Phanerozoic volcanic rocks with abundant xenoliths are widespread, making it an ideal area to study the formation and evolution of continents. New analyses of U-Pb ages and Hf isotopes in zircon were obtained for lower crustal xenoliths from four localities including the Paleozoic Yingxian lamproites, and the basalts of Pingquan (Paleocene), Hebi and Nushan (Neogene). Published ages and compositions of lower crustal and upper mantle xenoliths from the NCC are synthesized to constrain the accretion and reworking processes that have affected the deep lithosphere beneath the craton. The peridotite bodies within the Dabie-Sulu ultrahigh-pressure (UHP) belt, along the southern edge of the NCC, are compared with the xenolith peridotites to constrain early Mesozoic dynamics. The oldest components of the NCC may be similar to 4.0 Ga old. The craton experienced complex accretion and reworking processes in its deep lithosphere, accompanied by the formation (or aggregation) and differentiation of the ancient continental nucleus. The small size of the NCC, compared with many other cratons worldwide, made it more susceptible to the effects of marginal subduction and collision with surrounding blocks. The subcontinental lithosphere mantle (SCLM) was generally coupled with the lower crust through the Paleozoic, while decoupling occurred in late Mesozoic-Cenozoic time, except locally (such as the Neoarchen lower crust and SCLM in Hebi), suggesting strong interactions between the asthenosphere and the lithosphere (both upper mantle and lower crust) in Phanerozoic time. In the lower crust, the ancient components of the craton were re-worked in Paleoarchean (3.80-3.65 Ga) time. The craton also experienced two important accretionary episodes, in the Neoarchean (2.8-2.5 Ga) and the Paleoproterozoic (2.3-1.8 Ga). Asthenospheric upwelling in Neoproterozoic time (0.6 Ga) locally modified the lower crust. Subduction and collision of the surrounding blocks, such as the Yangtze Craton (YC), in Paleozoic and in early Mesozoic time also strongly modified the lower crust, especially along the cratonic margins. Accretion and modification of the lower crust during late Mesozoic-Paleogene is obvious due to the addition of depleted-mantle materials (underplating). In the SCLM, the subduction of the YC in early Mesozoic time may have resulted not only in a lateral spreading along the southern margin of the NCC and destruction of the integrity of the lithosphere in the interior of the craton, but also in mantle-wedge metasomatism by fluids and/or melts derived from the subducted continental crust. The initial destruction generated irregular channels for the subsequent upwelling of the asthenosphere induced by subduction of the Pacific plate (major lithospheric thinning). Since the late Mesozoic, cooling of the upwelled asthenosphere to form newly accreted lithosphere (similar to 125-100 Ma) has caused slight lithospheric thickening: the end result has been the wholesale replacement of the lithospheric mantle (thus SCLM accretion), but an overall lithospheric thinning. (C) 2012 Elsevier B.V. All rights reserved.