Genesis of ca. 850–835 Ma high-Mg# diorites in the western Yangtze Block, South China: Implications for mantle metasomatism under the subduction process

Abstract

High-Mg# (molar 100 × Mg/(Mg + Fe)) diorites can provide significant insights on mantle metasomatism under a subduction zone. Here we investigate the genesis of the Neoproterozoic Shuilu high-Mg# diorites in the western Yangtze Block to evaluate the role of subduction-related fluids and sediment melts acting on mantle sources during the subduction process. Zircon U–Pb dating results display new weighted mean 206Pb/238U ages of 850.1 ± 1.7 Ma, 840.9 ± 2.4 Ma, and 836.6 ± 1.9 Ma for these high-Mg# diorites. They are metaluminous and calc-alkaline rocks, and characterized by moderate SiO2 contents (57.08–61.12 wt%) and high MgO contents (3.36–4.30 wt%) and Mg# values (56–60). The relatively low initial 87Sr/86Sr ratios (0.703406 to 0.704157) and highly positive whole-rock εNd(t) (+3.26 to +4.26) and zircon εHf(t) values (+8.43 to +13.6) imply that these rocks were predominantly sourced from depleted lithospheric mantle. These high-Mg# diorites also show enrichment of light rare earth elements and large ion lithophile elements (e.g., Rb, Ba, K, and Sr) as well as depletion of high field strength elements (e.g., Nb, Ta, Zr, and Hf), resembling a typical arc magma affinity. The highly variable Ba contents and Rb/Y, Th/Ce, Th/Sm, Ba/La, and Th/Yb ratios indicate significant incorporation of subduction-related fluids and sediment-derived melts into the primary mantle source. We therefore propose that the ca. 850–835 Ma high-Mg# diorites investigated in this study were formed by partial melting of a metasomatized mantle source enriched by subduction fluids and sediment melts. Our new data, in conjunction with numerous studies of metasomatized mantle magmatism from the western Yangtze Block, suggest that the Neoproterozoic mantle sources beneath the western Yangtze Block were metasomatized by subduction-related compositions involving slab fluids, sediment melts, and oceanic slab melts during the subduction process.