Hafnium isotopic constraints on crustal assimilation in response to the tectono–magmatic evolution of the Okinawa Trough

Abstract

Crustal assimilation and the input of subducted sediment in a subduction zone play vital roles in generating arc and back-arc lavas along a continental margin. However, distinguishing the impacts of these two processes on lava geochemistry is not straightforward or well understood. In this paper, we present new Hf–Nd–Sr–Pb isotopic data for 20 igneous rocks ranging in composition from basalt to rhyolite, which when combined with data from the literature allow us to fingerprint the contributions of crustal and slab-derived materials along the Okinawa Trough (OT). Consistent with previous results, the basaltic lavas were generated by melting of an Indian-type mantle source metasomatised by slab-derived fluids and subducted pelagic/detrital sediments. We confirm that Hf behaves as a non-conservative element during subduction, constrained by Hf-Nd isotopic systematics. Our new data suggest that crustal assimilation processes worked independently of the extent of magma differentiation but were determined by the stage of tectonic development in individual regions. In the southern OT (seafloor spreading stage), crustal assimilation is insignificant, and the addition of subducted sediment is 0.4%–1.5%. In the middle OT (crustal rifting stage), the basaltic and rhyolitic suites do not show distinctive Hf–Nd–Sr–Pb isotopic compositions, and both record moderate crustal assimilation (<10%) and < 0.6% addition of subducted sediment. In contrast, in the southwestern OT (crustal extension stage), the basaltic and dacitic lavas record the highest involvement of 10%–40% crustal material. We conclude that the various stages of back-arc development exerted first-order controls on the extent of crustal assimilation in the OT and had marked effects on the geochemical and isotopic compositions of OT lavas.