Abstract
Abstract Slab material transfer processes in continental arcs can be challenging to decipher because magmas are often characterized by significant contributions from continental material. In this study, we identified a Prototethyan continental arc (419–418 Ma) that is now located in the Dazhonghe area of the southeast Tibetan Plateau, which, based on Sr-Nd-Hf-O-Si isotope relationships, implies no detectable continental material contributions. The Dazhonghe arc rocks display much lower δ30Si values than modern arc rocks and average mantle; this is best explained by subduction of sponge-rich marine sediments, which are thought to have been the dominant marine organisms during the Neoproterozoic and early Paleozoic. Our mixing calculations reveal that only bulk mixing among sponge-rich sediments, altered oceanic crust (AOC), and the depleted mantle would be capable of accounting for all the Sr-Nd-Hf-O-Si isotope compositions. This finding implies that the Dazhonghe arc magmas were generated by melting of a mélange that formed at the slab-mantle interface. The Dazhonghe continental arc is the first for which mélange melting has been confirmed.
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