Simulation of melt/fluid metasomatism and magma recharge and mixing processes in subduction zones: Evidence from the cognate gabbro and gabbroic diorite in the North Qilian orogen (Northwest China)

The arc-type characteristics of subducted crustal material incorporated into the mantle are closely related to metasomatism by melts and fluids derived from subducted oceanic crust. However, the specific compositions and proportions of melts and fluids from different end members of oceanic crust that metasomatize the mantle, as well as the subsequent processes of magma ascent, emplacement, and the evolution of mineral crystallization, remain inadequately understood. We conducted a detailed investigation of the major and trace element compositions of clinopyroxene, amphibole, and plagioclase in gabbro and gabbroic diorite from the North Qilian orogen in Northwest China to quantify the mechanisms of melt-fluid metasomatism, reveal the nature of the mafic magmatic arc source, and elucidate the processes of recharge and mixing of cognate magmas. Zircon U-Pb dating results indicate that the mafic intrusions in Zhamashi have emplacement ages of 505−481 Ma. The Sr-Nd isotopic compositions are relatively depleted, with (87Sr/86Sr)i ratios ranging from 0.7047 to 0.7086, and εNd(t) values ranging from 0.62 to 2.63. Trace elements display arc-related characteristics that are closely associated with the subduction of oceanic slab, which suggests that the Zhamashi mafic magmatic rocks originated from a mantle source metasomatized by aqueous fluids and hydrous melts derived from subducted North Qilian oceanic basaltic crust and sediments. Amphibole thermobarometer results indicate that the temperature for magma crystallization ranged from 940 °C to 994 °C, with pressure ranging from 417 MPa to 774 MPa, which corresponds to depths of 17.4 km to 29.5 km. The oxygen fugacity ranged from +1.3 to +1.9. The water content for melt varied from 5.3 wt% to 8.0 wt%, which is indicative of the typical heating and hydrous melting processes associated with contributions from subducting slab-derived fluids and melts. Complex mineral zoning structures and compositional variations record multiple episodes of cognate magma recharge and mixing during magma evolution, providing new insights into the dynamic processes that occur in subduction zone environments. Simulation results suggest that 5%−12% of oceanic crust-derived fluid and 0.5%−1.3% of sediment-derived melt reacted with garnet-phase lherzolite in the mantle wedge to form the metasomatized mantle, followed by 21% partial melting of the mantle source. These findings underscore the significance of melt-fluid metasomatism and multiple episodes of cognate magma recharge for understanding the genesis and diversity of arc magmatic rocks in subduction zones.