Niobium-enriched basalts: Partial melting of a sediment-metasomatised mantle source in subduction zones?

Crust-mantle interaction and arc magmatism in subduction zones record the global recycling of elements from Earth's surface to its interior. Nb-enriched basalts are specific to arc settings and are generated through the partial melting of heterogeneous mantle sources or mantle wedge metasomatised by subducted slab-derived melts. However, it is unclear what role subducted sediments play in the formation of Nb-enriched basalts. We present the first report of Cambrian Nb-enriched basalts at A'qiang and Pulu in the Western Kunlun Orogen (NW China), which might have formed above a subducted ridge-related slab window. The A'qiang basalts (Nb = 6.7-7.7 ppm) exhibit E-MORB-like geochemical characteristics, with positive Nb and Ta anomalies, high Mo/Ce ratios (-0.02), and positive epsilon Nd(t) (4.9-5.0) and low 898/95Mo (-0.74%o to-0.21%o) values. These basalts were generated through the partial melting of sub-slab asthenosphere modified by minor altered ocean crust and/or marly sediments. In contrast, the Pulu basalts (Nb = 10.9-13.7 ppm) are characterised by enrichment in light rare earth elements (LREEs), high Th/Ce (>0.1) and low Mo/Ce (-0.001) ratios, negative epsilon Nd(t) (-1.5 to-1.1) and mantle-like zircon 818O values (4.25%o-5.69%o), and high 898/95Mo values (-0.03%o to +1.17%o). The Pulu Nb-enriched basalts were derived from the partial melting of sub-slab asthenosphere modified by moderate siliciclastic sediments. Therefore, we propose that subducted sediments play a vital role in the transport of high field strength elements back into the mantle and their recycling into the crust, for example through the formation of Nb-enriched basalts.