Subduction-zone metamorphism is considered to be a major chemical filter for both the arc magmatism and mantle compositional heterogeneity. To understand the element transport during this process, we conducted a petrographic and geochemical study of the bulk-rock blueschists and eclogites from the western Tianshan ultrahigh-pressure metamorphic belt, northwest China. By examining correlations among incompatible elements, we show that high field strength elements (HFSEs), rare earth elements (REEs), Th, and U are relatively immobile, whereas Pb and Sr are mobile in both basaltic and sedimentary protoliths during subduction-zone metamorphism. K, Rb, Cs, and Ba are mobile in rocks of basaltic protolith but immobile in rocks of sedimentary protolith because of the presence and persistent stability of white mica throughout their petrologic history. The commonly observed enrichment of some immobile elements (e.g., U, light [L] REEs) in arc magmas may thus not be caused by subduction-zone aqueous fluids.
The lack of Rb/Sr-Sm/Nd correlation in these metamorphosed rocks is inconsistent with the observed first-order Sr-Nd isotope correlation in oceanic basalts, suggesting that the residual ocean crust that has undergone subduction-zone metamorphism cannot be the major source material for oceanic basalts, as widely believed, although it can contribute to mantle compositional heterogeneity in general.