The neodymium isotopic and geochemical composition of serpentinites from ophiolitic assemblages in the Qilian fold belt, northwest China

Serpentinites are a common component of ophiolitic assemblages in the Qilian fold belt. Samples associated with MORB-like basalts in the northern low-grade blueschist belt of the North Qilian terrane have lherzolitic and pyroxenitic compositions, high εNd(T) values of +5.5 to +6.9, and likely represent residues from melting along spreading ridge segments. Despite evidence for incompatible element enrichment by hydrothermal fluids which had interacted with the ocean floor sediment pile, samples at the Ayougou locality retain original isotope Sm-Nd systematics and yield a Silurian age of 422±20 Ma with εNd(T)=+6.8. Serpentinites in the southern high-grade blueschist belt of the North Qilian terrane and from the Lha-Jyi terrane of the South/Central Qilian belt are associated with volcanic arc rocks, and include lherzolitic, harzburgitic and dunitic compositions characterised by low εNd(T) values of +3.8 to −2.2. Harzburgitic samples from these localities display U-shaped REE profiles, and weakly developed negative Nb-anomalies superimposed on incompatible element enrichments, and are interpreted as remnants of oceanic lithosphere metasomatised by slab-derived fluids in a supra-subduction zone environment. Anomalous enrichments in the mobile elements U, Sr and Ba in serpentinites throughout the Qilian belt may be attributed to post-obduction alteration by groundwater.