The origin of Cenozoic continental basalts in east-central China: Constrained by linking Pb isotopes to other geochemical variables

Cenozoic continental basalts in east-central China are characterized by OIB-like trace element patterns with more depleted to less enriched Sr–Nd isotope compositions. Such geochemical signatures are attributable to variable contributions to their mantle sources from crustal components in the oceanic subduction zone. A combined study of basalt Pb isotope variations with other geochemical variables indicates that four mantle and crustal components were involved in the basalt petrogenesis. Model calculations verify the geochemical transfer from the subducted crustal components to the mantle sources. The depleted MORB mantle component is indicated by the depleted Sr–Nd isotope compositions of basalts. Relatively high 206Pb/204Pb and low Δ8/4 ratios are ascribed to contributions from the igneous oceanic crust with high U/Pb and low Th/U ratios, low 206Pb/204Pb and high Δ8/4 ratios are ascribed to the lower continental crust, and high 206Pb/204Pb and high Δ8/4 ratios are linked to the seafloor sediment. This generates different compositions of mantle sources for these OIB-like continental basalts. The basalts with the most depleted Sr–Nd isotope compositions show Pb isotope compositions distinct from Pacific MORB but similar to Indian MORB, suggesting the occurrence of Indian type asthenospheric mantle beneath the continental lithosphere of eastern China. The depleted MORB mantle would be metasomatized by the three crustal components at the slab-mantle interface in oceanic subduction channel, generating the mantle sources that are enriched in melt-mobile incompatible trace elements and their pertinent radiogenic isotopes. Nevertheless, the crustal components were not directly incorporated in the forms of crustal rocks into the mantle sources, but underwent partial melting to produce the felsic melts that predominate the composition of those trace elements and their pertinent radiogenic isotopes in the basalts. As such, the depleted MORB mantle component was contributed to the mantle sources in the form of peridotite, dominating the composition of major elements in the basalts. In is inferred that the westward subduction of Pacific slab beneath the North China Craton would lead not only to replacement of the ancient refractory lithospheric mantle by the depleted MORB mantle but also to incorporation of the crustal components into the mantle sources of continental basalts. Therefore, a series of processes from subduction, anatexis and reaction to storage and heating (SARSH) would be operated for the origin of continental basalts.