Hf–Nd–O isotopic evidence for melting of recycled sediments beneath the Sulu Orogen, North China

Abstract

The mantle source of late Mesozoic mafic magmas in the southeastern North China Craton is generally considered to have been enriched by deeply subducted continental lithosphere of the Yangtze Block. The mechanisms through which enrichment occurs are still unclear. Here we document an Early Cretaceous (zircon U–Pb age at 115±2Ma) dioritic intrusion with some geochemical features similar to sanukitoids (i.e., Al2O3>17.5%, Ba=1692–1819ppm, Sr=1042–1111ppm and δ18OZircon=6.2±0.2‰) from the Sulu Orogen in North China. Compared with the regional older (120–130Ma) mafic magmas (dykes), the diorite has less radiogenic Sr (87Sr/86Sr(i)~0.7075), higher εNd(t) (−9.0 to −8.6) with less radiogenic Hf than Nd (ΔεHf(t)=−6.1 to −5.2; ΔεHf(t)=εHf(t)−1.36×εNd(t)−2.89), signatures typically observed from continental zircon-bearing sediments. The primary magma of the dioritic intrusion was probably derived from a mantle enriched by the addition of a significant volume of a restitic component of recycled sediment that had been previously melted to leave residual zircon. Combined Hf–Nd–O isotopic data for the Rushan dioritic intrusion and the earlier mafic rocks requires the involvement of recycled sediments or their melts, incorporated into the lithospheric mantle as lenses or metasomatic veins, to produce a pudding-cake structure beneath the Sulu Orogen.