Recycled Crustal Components in the Depleted Mantle Source: Evidence from Early Cretaceous Mafic Rocks in the Southern Margin of the North China Craton

Abundant subducted materials are recycled into the deep mantle during the plate subduction process and are further reflected in the postcollision mafic rocks. The Early Cretaceous Mid-Oceanic-Ridge-Basalt (MORB)-like mafic rocks in the southern margin of the North China Craton (SNCC) preserve significant information on recycled materials. These rocks provide a window for discussing the crust-mantle interaction and tracing the history of subduction. The studied rocks comprise the Jingwan metadiabase, Putaozhuang, Nianpanzhuang, and Niujiaowan metahornblende gabbro, with Early Cretaceous ages (140-129 Ma). They show large ion lithophile element enrichment in both the Enriched-MORB and Normal-MORB rare earth element patterns. They are characterized by slightly depleted isotopic compositions, high MgO, Cr, Ni and Mg-# values as well as the low SiO2, suggesting a depleted mantle source affinity. The enriched NdHf (epsilon(Nd)(t) = -14.50 to -13.47; epsilon(Hf)(t) = -9.27 to -8.82) and unradiogenic Pb isotopes imply that Jingwan samples originated from a depleted mantle source metasomatized by lower continental crustal materials. Their relatively enriched zircon epsilon(Hf)(t) values (-15.8 to -1.6) and delta O-18 values (4.82 parts per thousand-8.77 parts per thousand) along with their Paleoproterozoic to Neoproterozoic inherited zircons also argue for the contribution of low continental crustal materials from the North China Craton. In contrast, Putaozhuang, Nianpanzhuang, and Niujiaowan samples have positive whole-rock epsilon(Nd)(t) values (4.08-5.20) and epsilon(Hf)(t) values (3.23-5.62). Combined with the existence of Paleozoic zircons, high zircon epsilon(Hf)(t) values, and high Nb/U ratios, these materials suggest that they originated from a depleted mantle source metasomatized by juvenile crust and terrigenous sediments. This is corroborated by variable zircon delta O-18 values (5.57 parts per thousand-9.63 parts per thousand) and mantle-like delta O-18 values (5.08 parts per thousand-5.60 parts per thousand) of Paleozoic inherited zircons from Niujiaowan samples. The distinct geochemical characteristics of Early Cretaceous mafic rocks are induced by the incorporation of different components from subducted crust and sediments into the depleted mantle source. Paleo-Pacific Plate far-field effects controlled the intense lithospheric extension and thinning of the Early Cretaceous MORB-like rocks in the SNCC, which were placed in an immature back-arc basin setting.