Geochemical signatures track the evolution of the northwest North China Craton upper mantle

Earth’s upper mantle, the layer above the seismic discontinuity at ~410 km depth, exerts a primary control on the mass and energy inventory of our habitable environment. However, characterization of its structure and composition is notoriously difficult due to limited access to the deep Earth, especially in deep time. Basaltic rocks and their entrained xenoliths represent partial melts and fragments of the mantle, respectively; they can jointly serve as locality-specific and time-specific lithoprobes into Earth’s interiors. Systematic studies of lithoprobes from multiple localities at different timeslices may decipher the 4-D upper mantle evolution of targeted regions. The upper mantle of western North China Craton remains poorly constrained due to the scarcity of lithoprobes in this region, leaving an incomplete picture of the cratonic mantle’s destruction and renewal, and the related geodynamic environments. This thesis presents a comprehensive study on newly-found lithoprobes in the craton’s northwest (Langshan area) with explicit attention to the tectonic processes on its northern boundary; the new knowledge derived from this study is summarized below. 

Mantle xenoliths reported in this study are of two types, the Cr- and Al-pyroxenites. The former group, typically cutting mantle peridotites with diffuse contacts, is depleted in basaltic components (e.g., FeO, Na2O, and Al2O3, and incompatible elements) and Sr-Nd isotopic compositions; they are considered to form through reaction of peridotites with silica-rich asthenospheric melts in the Paleozoic. In contrast, the latter group usually are magmatically layered with crustal granulites and have relatively enriched element and isotopic ompositions; they are interpreted to be Late Mesozoic crustal cumulates from hypersthene-normative melts generated by interaction between asthenospheric and heterogeneous lithospheric mantle. Mantle domains with distinct radiogenic Pb isotopic compositions (207Pb/204Pb > 16.7, 206Pb/204Pb =~19) represented by these pyroxenites are shown to reflect lithospheric isolation from the convecting mantle at ~1.4 Ga. 

Basaltic rocks comprise xenolith-bearing alkali basalts (~89 Ma) and lamprophyre dykes (~81 Ma). Zircon xenocrysts from the basalts record vigorous Mesoproterozoic (~1.35 Ga) and Paleozoic (470–230 Ma) magmatic events. The basalts and lamprophyres have overlapping elemental and Sr-Nd-Pb-Mg isotopic compositions; both are characterized by low silica, high alkalis and incompatible elements, with trace-element patterns and Sr-Nd isotope ratios similar to those of ocean-island basalts. They are interpreted as partial melts of pyroxenite-bearing asthenosphere, triggered by small-scale convection beneath corrugated lithospheric roots. Their isotopically light Mg (δ26Mg = -0.391 to -0.513) combined with low Ca/Al and high Zr/Hf may ii reflect the breakdown of subducted carbonates and transfer of their residua to the melt sources as Mg-isotopically light pyroxenites. 

Integration of data for peridotite xenoliths and the regional geology suggests that the upper mantle of the North China Craton probably evolved together with the birth and death of the Paleo-Asian Ocean, including subcretion of fertile mantle from cooled asthenosphere during ocean opening and melt movements during subduction and closure. This study shows the great potential of the upper mantle at continental margins to reveal the secular interaction between the subducting oceanic slab, convecting mantle wedge and overlying lithosphere.