Key role of recycled terrigenous sediments in generating continental arc magmatism in the West Qinling orogen, China

Abstract

Substantial quantities of heterogeneous sediments are transported into the mantle at subduction zones. However, geochemical signatures linked to recycled terrigenous sediments are almost absent in MORB and are only sporadically observed in some hotspot volcanoes. Therefore, identifying the provenance of the subducting sediments and their fate in the mantle is critical for unraveling the geochemical evolution of the silicate earth. Here we present a comprehensive study of the Triassic ultramafic-mafic igneous rocks from the West Qinling orogen, China. These rocks display arc-like trace element features, weakly enriched Sr–Nd–Hf isotopes, low Ba/Th, Ba/La, Pb/Ce and U/Th ratios, and high Th/La, Th/Yb, Th/Nd and La/Sm ratios. These characteristics suggest that they originate from a mantle source predominantly metasomatized by the subducting sediment-derived hydrous melts. In addition, their high Th/La, Th/Yb and Hf/Sm ratios, paired with low Lu/Hf ratio and significantly low ε_Hf(t) values, closely resemble those of terrigenous sediments, indicating a primarily terrigenous origin. Furthermore, these rocks exhibit lighter Li isotope ratios than MORB, similar to the arc rocks whose mantle sources have incorporated isotopically light terrigenous sediments. By comparing the geochemical features of these ultramafic-mafic igneous rocks with seafloor sediments, we propose the rocks in the West Qinling orogen are derived from a mantle wedge predominantly enriched by subducting terrigenous sediments. Their geochemical features are similar to those of modern continental arcs, suggesting the contribution of different sediment types largely depends on the location of the island arc (oceanic vs. continental). Moreover, much of the subducting sediments may rapidly return to the surface during continental arc magmatism. Therefore, this study provides new insights into the origin and fate of subducting terrigenous sediments in the mantle, as well as their role in generating mafic magmatism in continental arc systems.