Insight of enriched basalts into the nature and evolution of mantle lithosphere beneath craton margins

Refractory lithospheric deep roots are the cornerstone for the prolonged stability of cratons and mantle xenoliths are normally the key targets for study on the evolution of such deep roots. In regions with few mantle xenoliths, the basalts enriched in radiogenic isotopic compositions due to marked lithospheric mantle contribution are crucial to unmask the lithospheric mantle evolution based on a comprehensive study involving petrology, geochemistry and thermodynamic modelling. Here, the Early Cretaceous basalts from the northwest North China Craton with few mantle xenoliths are taken as an example to show the significance of enriched basalts on the study of lithospheric mantle. These basalts are characterized by high silica and alkali contents (SiO2=45.8-59.8 wt.%, K2O+Na2O=4.81-9.88 wt.%), arc-type trace-element patterns and enriched radiogenic isotope compositions (e.g., epsilon Nd=-2.64--12.88,87Sr/86Sr=0.7063-0.7093). The TiO2 and FeO (total) contents are higher than those of natural and experimental melts from refractory mantle peridotite but comparable to those of partial melts of fertile mantle rocks. The high contents of fluid-loving elements (e.g., Rb and Ba) suggest source metasomatism by aqueous fluids. Combined with thermodynamic modelling and regional tectonic history, these enriched basalts likely record simultaneous decompression melting of asthenosphere and low-extent melting of thin and fertile lithospheric mantle fluxed by aqueous fluids from the subducted Paleo-Asian oceanic slab. The newly unmasked lithospheric mantle under the western NCC contrasts with the coeval thick and refractory one supporting the eastern NCC, and highlights that the craton destruction, especially the loss of its ancient refractory mantle root, should take place in a diachronous manner related to the craton-girded subduction episodes. Our study illustrates the potential of enriched basalts to recover the nature and evolution of mantle lithosphere beneath craton margins and associated tectonic histories.