EVIDENCE FOR CRUSTAL CONTAMINATION IN INTRA-CONTINENTAL OIB-LIKE BASALTS FROM WEST QINLING, CENTRAL CHINA: A RE-OS PERSPECTIVE

The magnitude of crustal contamination in intra-continental OIB-like basalts is commonly difficult to assess using traditional approaches. However, the Re–Os isotope system is a very sensitive proxy of the crustal component during formation of basalts. Here we report Sr–Nd–Pb–Os geochemical and isotopic data from the Cretaceous Duofutun Volcanic Rocks (DVRs) of West Qinling, central China. The DVRs exhibit low Os abundances (4.46–42.02 ppt) with Re abundances ranging from 45.58 to 812.98 ppt, variably high Re/Os (3–126), and high initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.2112–0.6784). We propose that the degree of partial melting is responsible for the first-order control on the variation of Os concentrations. Even the least radiogenic sample still possesses a higher initial ¹⁸⁷Os/¹⁸⁸Os ratio than the primitive upper mantle, implying a non-negligible contribution from high Re/Os and ¹⁸⁷Os/¹⁸⁸Os reservoir(s). Both the crust and pyroxenite-rich mantle are reliable candidates, but the latter cannot be reconciled with the Os mass balance between pyroxenite and ambient peridotite. Consequently, we contend that the DVRs were contaminated by crustal materials en route to the surface. Assimilation/fractional crystallization modeling manifests the observed isotopic variations can be produced by minor assimilation of the Proterozoic mafic lower crust beneath West Qinling. On a global scale,we collected a great amount of Re-Os data related to basalts. We found that the intra-continental OIB-like basalts show a relatively wider range of ¹⁸⁷Os/¹⁸⁸Os ratios in comparison with MORB and OIB, confirming the continental crust can have a higher concentration of radioactive Os, which is induced by time-integrated growth.