Geochemical evidence for incorporation of subducting sediment-derived melts into the mantle source of Paleozoic high-Mg andesites from northwestern Tianshan in western China

Abstract Although high-Mg andesites (HMA) have attracted increasing attention due to their unique geochemical composition and important geological significance, there is no consensus on petrogenesis. The present study indicates that the subducting terrigenous sediment-derived hydrous melts were incorporated into mantle source of Paleozoic HMA in northwestern Tianshan, western China. These are composed basaltic andesite andesite. They generally exhibit arc-type trace element distribution patterns weakly enriched Sr-Nd-Hf isotope compositions. Some them show remarkably higher Ba/La, Ba/Th, Rb/Nb, U/Th ratios than normal mid-ocean ridge basalts (MORB). Furthermore, most characterized by Th/Nb, Th/Yb, Th/Nd but lower Nb/U relative MORB, similar those sediments. Whole-rock Nd isotopes covary with Rb/Nb Th/Yb ratios, indicating contributions from both oceanic crust-derived aqueous solutions melts. Together high zircon δ18O values, it appears target contains addition solutions. In studied HMA, exhibits contents large-ion lithophile elements light rare earth more enrichments differences indicate would contain sedimentderived As a consequence, relatively Si-rich Si-poor pyroxenite sources respectively generated as This qualitative interpretation verified quantitative modeling transfer crust wedge. Model calculations ~3% 4%–12% wedge peridotite can account for compositions HMA. Therefore, Tianshan provide evidence crust-mantle interaction during subduction Paleozoic. such, play dominant role lithochemically fertile, geochemically thus origin above zones.