Petrogenesis of Cenozoic basalts in central-eastern China: Constraints from Re-Os and PGE geochemistry

- Cenozoic basalts in CE China have low-Ca, high-Ca alkali and tholeiitic varieties.
- Low-Ca alkali basalts have low 187Os/188Os and flat chondrite-normalized PGE patterns.
- High-Ca alkali and tholeiitic basalts have variable 187Os/188Os and Pd/Ir ratios.
- Os isotopic variations reflect source heterogeneity rather than crust contamination.
- Highly suprachondritic Os indicates that the mantle source contains pyroxenites.

We present Re-Os and PGE (platinum group elements), together with major-, trace-element and Sr-Nd isotope data, to further constrain the petrogenesis for the Cenozoic basalts in central-eastern China. The basalts include low-Ca alkali, high-Ca alkali and tholeiitic varieties. Most low-Ca alkali basalts have unradiogenic 187Os/188Os (mostly < 0.14), flat chondrite-normalized PGE patterns and depleted Sr-Nd isotopic compositions, indicating that they probably originated from a depleted mantle source with insignificant shallow-level crustal contamination. Minor high-Ca alkali/tholeiitic basalts have low PGE concentrations, particularly Os (down to 0.0056 ppb), and highly suprachondritic initial 187Os/188Os ratios (up to > 0.4), indicating involvements of sulfide/PGE-alloy segregation-AFC (assimilation-fractionation-contamination) processes. Most tholeiites and high-Ca alkali basalts have relatively high Os concentrations, highly variable and suprachondritic Os isotopic compositions, high Pd/Ir ratios and enriched Sr-Nd isotopic compositions. Modeling results reveal that the Os isotopic variations in these basalts cannot be ascribed to crustal contamination during ascent but mainly reflect source characteristics. Combined with their low CaO, low Mg#, high FC3MS values (FeOT/CaO-3 × MgO/SiO2, all in wt.%), and high Fe/Mn signatures, a large portion of pyroxenite is inferred as an enriched component in the mantle sources to account for their highly suprachondritic Os isotopic compositions. The formation of the pyroxenites was probably due to the subducted sediment-bearing Pacific oceanic crust or the recycled lower continental crust resulted from the collision between the Yangtze and North China blocks.