Lithium and molybdenum isotope systematic for Fushui complex and implications for the recycled oceanic crust in the source of arc magmatism

The origin and extent of crustal material in arc magmas are critical for tracing the crust-mantle interaction at subduction zones. Arc volcanic rocks often have a characteristic range of trace elements and isotopic compositions that constrain the influx of crustal material, such as hydrous fluids derived from slab dehydration, melts of the subducted plate's veneer sediments, or heterogeneous mélange. Especially, the traditional geochemical proxies struggle to investigate the contribution of tectonic melange into the enriched subarc mantle source. Lithium and molybdenum isotope compositions provide a potential means of tracing the different components to the mantle wedge in subduction zones.

Here, we report Li-Mo isotope compositions of the Fushui mafic complex in the Qinling orogen, central China, to trace their source nature and to constrain metasomatic component(s) transferred during the early Paleozoic subduction event. The Fushui mafic rocks exhibit typical arc-type trace element features and enriched radiogenic isotope compositions, suggesting a mantle source had incorporated subducted compenents. The Fushui rocks also exhibit a light Li isotope composition providing direct evidence for the contribution of dehydrated serpentinites and eclogite. The variable Mo isotope values further suggest the mantle source has been metasomatized by aqueous fluid and hydrous melt. This study, therefore, confirms the transportation of dehydrated oceanic plates from footwall to hanging wall and its critical role in the subarc mantle source. The interpretation provides a new perspective to study the recycling of subducted oceanic lithosphere and associated petrogenesis of arc magmatism.