Melting of mafic slab and mantle peridotite during ridge subduction of the Proto-Tethys Ocean (Qilian Orogen, NW China)

Abstract

Ocean-ridge subduction and the induced slab windows are important to understand the generation of arc magmatism and growth of juvenile crust both in present and ancient subduction zones. The physical conditions of oceanic ridges control the compositions of magmas during ridge subduction, and therefore magmatic composition can act as a marker of ridge subduction in ancient orogenic belts. Here we present the petrological, geochemical and geochronological studies for sodic rhyodacites and a concurrent ophiolite complex (basalt/dolerite-gabbro and harzburgite) within the North Qilian orogen, NW China. The sodic rhyodacites are low Sr/Y tonalite-trondhjemite with depleted SrNd isotopic compositions, suggesting derivation from melting of normal mid-ocean-ridge basalts (N-MORB) at 449 ± 2.9 Ma. Phase equilibrium and geochemical modeling confirm that the sodic rhyodacites formed via the partial melting of N-MORB sources at 3–5 kbar/780–840 °C. The basalts/dolerites exhibit similar geochemical characteristics to typical N-MORB rocks, and the gabbro gives zircon UPb age of 450 ± 2.5 Ma. The petrogenesis of the basalts/dolerites is similar to the “blow-torch” effect often seen during ocean-ridge subduction. The harzburgites are similar to abyssal peridotites in mineral compositions and have equilibrium temperatures of 1230–1460 °C, suggesting the obduction of a hot asthenospheric mantle on the continental margin. Our results show the integrated melting processes of the upwelling mantle peridotite and the subducted oceanic crust within the slab window during ridge subduction of the Proto-Tethys Ocean. These results are also significant in understanding the petrogenesis of low Sr/Y tonalite-trondhjemites, which are widely preserved in Archean to modern terranes.