Geochemical signature of a deeply subducted slab in the source of post-collisional magmatism: A case study from the Western Tianshan NW China

Abstract

The contributions of deeply subducted slabs to post-collisional magmatism are poorly constrained. Here, we present a study of the post-collisional, Early Permian (ca. 279 Ma), Hamisite basalts from the Western Tianshan NW China to trace the geochemical fingerprints of deeply subducted slab. The Hamisite basalts have arc-like trace element patterns but high Nb/Ta ratios (18.4−23.0). They have slightly enriched Sr−Nd isotopic compositions (87Sr/86Sr = 0.7048−0.7052; εNd[t] = 2.02−3.85), which indicates that the basalts were derived from an enriched mantle source. Extremely low δ7Li values (−5.76‰ to 0.20‰) imply the involvement of a deeply subducted slab (i.e., eclogites) in the source. Correlations among Nb/Ta and (Dy/Yb)N ratios, and rare earth element contents, indicate that the deeply subducted slab contained residual rutile and garnet but no epidote. Given that melting of eclogites requires an anomalously hot mantle source, we propose that mantle upwelling during post-collisional extension triggered melting of the deeply subducted slab. Our results indicate that melting of a deeply subducted slab can generate a metasomatized mantle source with high Nb/Ta ratios, which could be a hidden, high-Nb/Ta reservoir on Earth. Low δ7Li values and high Nb/Ta ratios could be a fingerprint of recycling of a deeply subducted slab.