Arc magmatism associated with steep subduction: Insights from trace element and Sr–Nd–Hf–B isotope systematics

Abstract

AbstractSubduction zones are the major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting. However, the nature of slab fluids associated with steep subduction remains largely unknown. To clarify this issue, we present an integrated study for Late Paleozoic (318–312 Ma) intermediate dykes from the Beishan orogenic collage, NW China. The dykes consist mainly of dioritic and granodioritic rocks. The dioritic dykes exhibit typical subduction‐like geochemical signatures, together with relatively high Mg#, high εNd(t) and εHf(t), and low initial Sr isotopes, suggesting that they originated probably from a subduction‐modified mantle. The granodioritic dykes exhibit high Mg#, high Sr/Y, La/Yb, and Na2O/K2O ratios, low Y and Yb contents, and mid‐ocean ridge basalt‐like Sr–Nd isotopes and high zircon εHf(t), similar to slab‐derived adakite, indicating that they were likely formed by partial melting of subducted oceanic crust. The coeval adakitic and normal dioritic dykes reflect a thermal anomaly that was probably caused by rollback of subducted oceanic slab. The dioritic dykes have δ11B values from −7.7 to −6.4‰, whereas the adakitic dykes have relatively high δ11B values from −6.9 to −4.4‰. The δ11B values of adakitic dykes are lower than those of typical altered oceanic crust, in agreement with the expected loss of 11B from subducted oceanic slab during early subduction. Results of a mixing model suggest that the mantle source of the dioritic dykes has been hybridized by 11B‐depleted fluids expelled from a highly dehydrated slab at deep depth, owing to the high‐angle dip of the subducting oceanic slab.