Cretaceous Nb-enriched basalts in the western Gan-Hang belt, South China: A response to slab window opening during the Paleo-Pacific ridge subduction

The role of recycled slab-mantle interaction in the formation of mantle-derived rocks during the Paleo-Pacific subduction remains to be tested. In this study Nb-enriched basalts (NEBs) are identified in Shanggao, western part of the Gan-Hang belt, South China. These volcanic rocks have 40Ar/39Ar ages of ∼90 Ma. They are tholeiitic to calc-alkaline in composition and have low SiO2 (50.1–53.4 wt%), high MgO (5.86–7.10 wt%), and Nb (10.7–14.3 ppm) contents. These NEBs have high Nb/La (0.91–0.99) and Nb/U (14.8–21.0) ratios and chondrite-normalized patterns with slightly enriched light rare earth elements (LREEs) ((La/Yb)N = 5.32–6.52). Their primitive mantle-normalized patterns are similar to E-MORB-like characteristics. These features indicate that these NEBs were derived from a mixed source between the asthenospheric mantle and slab-derived adakitic components in the garnet stability field. Their Sr-Nd-Pb-Hf isotopic compositions also reflect the contribution of the subducted oceanic crust including minor sediments in the source. These NEBs are spatially associated with adakitic rocks, A-type granites, and related mineral deposits in the Gan-Hang belt, clearly supporting a tectonic model in which the Cretaceous ridge subduction and slab window opening occurred during the enhanced slab rollback of the Paleo-Pacific plate. The melting of residual eclogites caused by episodic upwelling of the asthenospheric mantle can account for the formation of the NEBs in a back­arc extensional setting.