The rocks in the crustal section of the Oman ophiolite show an increasing input of a subduction component with time, most likely reflecting the generation of the ophiolite above a subducting slab. Field relations, new geochemical data, and Nd-Hf isotope data for felsic to mafic intrusive rocks in the mantle harzburgite from the Haylayn block in the Oman ophiolite suggest late magmatic events in a mantle wedge shortly before obduction of the ophiolite. Incompatible element contents and low εNd and εHf of the felsic rocks exclude differentiation from mafic magmas, but are consistent with an origin by partial melting of pelagic sediments similar to leucogranites in continental collision zones. These melts apparently mixed with mafic magmas resembling enriched late-stage lavas from the ophiolite. The leucogranitic intrusions into the mantle wedge confirm the transfer of melts of sediments from the subducted plate into the mantle at subduction zones. We suggest that the enrichment of Rb, K, and Pb observed in the Oman boninites is caused by addition of melts of sediments similar to those from the Haylayn block to the boninite source in the mantle wedge.