Deep lithosphere of the North China Craton archives the fate of the Paleo-Asian Ocean

Abstract

Understanding the formation, preservation and destruction of continental lithosphere in the paradigm of plate tectonics remains one of the “holy grails” of solid Earth Science. It is well recognized that the eastern North China Craton (NCC) experienced Mesozoic replacement of thick, ancient and refractory lithospheric mantle by a thin and fertile one, in response to the (Paleo-) Pacific subduction. However, the deep lithospheric responses to the long lifespan of the Paleo-Asian Ocean on the north of the craton are still poorly constrained. Based on a comprehensive review of the evolution of the NCC and the Paleo-Asian Ocean, we reappraise the available data on deep lithospheric (uppermost mantle and lower crust) xenoliths from northwestern NCC in conjunction with new UPb ages and trace elements of zircon xenocrysts, and bulk-rock Pb-isotope data for pyroxenite xenoliths and mafic dykes. The northwestern NCC, far from the tectonic regimes on the south and east, has great potential to preserve the cratonic responses to the evolution of the Paleo-Asian Ocean.The fertile lithospheric mantle underlying the ancient northwestern NCC is characterized by low concentrations of CaO in olivine (<0.1 wt%), TiO2 in spinel (typically <0.2 wt%), Wo component in orthopyroxene (<0.3), Ca/Al (<6), and high Na in clinopyroxene. These characteristics are distinct from those of refertilised cratonic mantle and oceanic mantle and resemble those of cooled asthenosphere. Lithospheric mantle domains with radiogenic Pb isotope compositions (e.g., 207Pb/204Pb > 16.7, 206Pb/204Pb = ~19) are recorded by Cr-pyroxenite and Al-pyroxenite xenoliths from the westernmost NCC. Such a radiogenic domain could be produced by lithospheric isolation from convecting mantle containing ancient recycled materials at ~1.4 Ga and was probably related to the opening of the Paleo-Asian Ocean. Zircon xenocrysts and crustal xenoliths from across the northwestern NCC record vigorous Mesoproterozoic (e.g., ~1.35 Ga) and Paleozoic (470–230 Ma) magmatic and thermotectonic events. The parental melts of the zircon xenocrysts are inferred to have been basaltic andesitic to andesitic in composition; the crystallization temperatures peak at ~900 °C, comparable to the equilibration temperatures of the lower crustal xenoliths. They are probably the deep crustal record of the opening and secular subduction of the Paleo-Asian Ocean. Late Cretaceous mafic rocks from the westernmost NCC, with elemental and SrNd isotopic compositions analogous to those of Cenozoic alkali basalts across eastern Asia, have relatively radiogenic Pb isotope compositions (206Pb/204Pb = 18.5–19). Similar radiogenic components are recognized in the Cenozoic basalts from the eastern NCC and the southeastern coast of China, and these signatures suggest the ubiquitous presence of recycled Paleo-Asian oceanic slab materials in the convecting mantle during the Late Mesozoic to Cenozoic.Collectively, the deep lithospheric modification of the northern NCC provides an archive of the fate of the Paleo-Asian Ocean, including the generation of fertile lithospheric mantle from cooled asthenosphere during its formation and melt infiltration during subduction and closure. This study shows the great potential of deep lithosphere at continental margins to reveal the secular interaction between the subducting oceanic slab, convecting mantle wedge and overlying lithosphere.