Neoproterozoic Oceanic Slab-Mantle Interaction: Geochemical Evidence from Mesozoic Andesitic Rocks in the Middle and Lower Yangtze Valley

The generation of continental arc andesites is generally attributed to subduction of oceanic slabs beneath continental margins, but the petrogenetic processes of andesites remain widely debated. In order to address this problem, a series of integrated geochemical studies were performed for Mesozoic andesitic volcanics and associated basaltic and dacitic volcanics from the Middle and Lower Yangtze Valley, South China. The results lead to proposition of a new model for the generation of andesites. Laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) zircon U-Pb dating yields consistent ages of Early Cretaceous for the formation of these volcanics, which are characterized by arc-like trace element distribution patterns showing significant enrichment in large ion lithophile element (LILE), Pb and light rare earth element (LREE) but depletion in high field strength element (HFSE) and heavy rare earth element (HREE). They also exhibit relatively enriched Sr-Nd-Hf isotope compositions, high radiogenic Pb isotope compositions and high zircon O isotope composition. Crustal contamination and magma mixing had insignificant contributions to the enriched compositions of these andesites. Instead, the enriched compositions were imparted by incorporating the subducted crust-derived materials into their magma sources. Despite their formation in the Late Mesozoic, their magma sources were generated through the crust-mantle interaction when the Cathaysian oceanic crust was subducted beneath the Yangtze craton in the Early Neoproterozoic. There are large amounts of subducted sediment-derived hydrous melts in the magma sources of continental arc andesites, in contrast to the limited amounts of aqueous solutions and hydrous melts in the magma sources of oceanic arc basalts. It is the hydrous melts that would chemically react with the overlying mantle wedge peridotite to generate mafic-ultramafic metasomatites. In the Early Cretaceous, these metasomatites underwent partial melting due to remote backarc extension owing to westward subduction of the Paleo-Pacific slab beneath the eastern China continent. Whereas partial melting of the ultramafic metasomatite produced basaltic melts, partial melting of the mafic metasomatite produced andesitic melts. In this regard, petrogenesis of both continental arc andesites and oceanic arc basalts shares two-stage processes, in which the first is the generation of mantle sources through subduction zone metasomatism and the second is the partial melting of mantle sources for mafic magmatism, with the first stage corresponds to the slab-mantle interaction in oceanic subduction zones.