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

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 Sr-Nd 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 degrees C. The basalts/ dolerites exhibit similar geochemical characteristics to typical N-MORB rocks, and the gabbro gives zircon U-Pb 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 degrees 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.

Automated summary

Ocean-ridge subduction and induced slab windows play a crucial role in the formation of arc magmatism and growth of crust. A study in Northwestern China found low Sr/Y rhyodacites and associated ophiolites, which were formed through partial melting of normal mid-ocean-ridge basalts and exhibited characteristics of blow-torch effect during ocean-ridge subduction. Harzburgites in the area were similar to abyssal peridotites and formed through the obduction of hot asthenospheric mantle. These findings provide important insights into the petrogenesis of low Sr/Y rhyodacites and shed light on the processes occurring during ridge subduction.