Nd-Sr isotopic and geochemical evidence on the tectonic setting of the Tadun Creek ophiolite, North Qilian fold belt, NW China

Metamorphosed mafic rocks of the Tadun Creek section of the low-grade blueschist belt in the North Qilian terrane include metabasalt, blueschist and metagabbro representing remnants of an Ordovician ocean basin. Metabasalts and blueschists both resemble N-MORB in composition and have εNd(450Ma)=+7.3 to +9.3. Metagabbros are characterised by elevated Al₂O₃, lower εNd (+7.1 to +7.3), lower Ti/V (10-20) and higher Cr/Y (>15) ratios suggesting a relationship to arc volcanism. Market depletion in HFSE abundances in the metagabbros suggests an origin as a supra-subduction ophiolite rather than a back-arc basin sequence, though the latter is not precluded.