Geochemical evidence for interaction between oceanic crust and lithospheric mantle in the origin of Cenozoic continental basalts in east-central China

Abstract

Cenozoic continental basalts from east-central China have been analysed for whole-rock major and trace elements, Sr–Nd–Hf isotopes, and mineral O isotopes. The basalts are alkalic to tholeiitic in composition, but are dominantly alkalic. They are characterized by OIB-like patterns of trace element distribution, with no depletion in Nb and Ta but with negative Pb anomalies on primitive mantle normalized diagrams. In addition, they have high Fe/Mn ratios and subchondritic Nb/Ta ratios. Sr–Nd–Hf isotope compositions indicate predominant contributions from young subcontinental lithospheric mantle (SCLM). Some phenocrysts of clinopyroxene and olivine have lower O isotope ratios than normal upper mantle, suggesting involvement of low δ 18O melts derived from dehydrated oceanic basalt that experienced high-T seawater-hydrothermal alteration during MORB-type magma eruption. Such low δ 18O melts are unlikely to be transported through the convective asthenosphere because of its rapid O isotope homogenization, but likely to directly metasomatize the overlying peridotite to form pyroxenite. Oxygen isotope disequilibrium between olivine and clinopyroxene suggests a maximum timescale of < 1 ka for melting, transport and eruption during basaltic magmatism. Therefore, we propose a model of oceanic crust-lithospheric mantle interaction for petrogenesis of the continental basalts, in that they were principally derived from near-solidus partial melting of pyroxenites in the juvenile SCLM. Low-angle subduction of the oceanic crust beneath the continental lithosphere during the Early Mesozoic would have not only delaminated the ancient SCLM into the asthenosphere to result in the mantle heterogeneity, but also transform the overlying mantle wedge to the juvenile SCLM beneath the thinned ancient SCLM. Partial melting of the dehydrated oceanic metabasalt and metasediment during the Late Mesozoic would generate adakitic and felsic melts, respectively, metasomatize the overlying juvenile SCLM peridotite to form isotopically depleted silica-deficient to enriched silica-excess pyroxenites. As a result, SCLM beneath the eastern China was stratified in the Cenozoic, with the juvenile SCLM underlying the ancient SCLM but overlying the pyroxenites. Lithosphere rifting during the Cenozoic would cause the melting of the pyroxenites, giving rise to the alkalic to tholeiitic basalts. Therefore, the Cenozoic continental basalts provide the petrological record of melt-peridotite reactions in response to the asthenosphere-lithosphere interaction during the Mesozoic thinning of continental lithosphere.