Variable slab-mantle interaction in a nascent Neoproterozoic arc-back-arc system generating boninitic-tholeiitic lavas and magnesian andesites

Subduction zones are major sites of elemental recycling on Earth, and they are pivotal in exploring the mechanisms of crustal growth and differentiation. The late Neoproterozoic mafic volcanic suite from the southeastern Gorny Altai terrane represents magmatic products of the nascent Kuznetsk-Altai intra-oceanic island arc southwest off the Siberian continent. These rocks preserve key information on the subarc mantle source and slab-mantle interaction during the early phase of ocean-ocean subduction, and therefore they might shed light on the geodynamic evolution of subduction zones worldwide. The mafic volcanic suite can be geochemically classified into four rock types, i.e., group I tholeiitic lavas with relatively high TiO2 contents, group II tholeiitic lavas with comparatively lower TiO2, boninitic rocks, and magnesian andesites. Trace-element and Nd isotopic compositions demonstrate that group I tholeiitic lavas possibly formed by decompression melting of a fertile asthenospheric mantle source with subtle involvement of slab-derived components. In contrast, the precursor magma of the boninitic rocks was probably derived from depleted clinopyroxene-poor lherzolite metasomatized by slab-derived aqueous fluid and minute amounts of hydrous siliceous melt from the subducted sediments. The high Ti/Zr and low Zr/Sm ratios of these rocks further suggest that garnet was possibly a residual phase during partial fusion of the subducted sediments. These data and their close spatial association suggest that group I tholeiitic lavas and the boninitic rocks possibly formed in a back-arc-related setting. Group II tholeiitic lavas show geochemical compositions intermediate between those of the boninitic rocks and the group I tholeiitic ones. We infer that group II either represents island-arc tholeiites that were generated during the proto-arc stage, or they were formed by mixing of precursor magmas of group I tholeiitic lavas and the boninitic rocks. Geochemical modeling reveals that the magnesian andesites probably formed more adjacent to the trench, with significant contribution of a fertile asthenospheric mantle source and sediment-derived siliceous melt with amphibole in the residue. Our data suggest that initiation or propagation of a back-arc basin in response to slab rollback was a possible geodynamic cause for the volcanic activities in the southeastern Gorny Altai terrane, where the upwelling asthenosphere and variable components from the subducted slab and overlying mantle wedge all made significant contributions to a potentially nascent arc-back-arc system. The available data for similar rock associations worldwide imply that a similar tectonic scenario may prevail during the early phase of intra-oceanic island arcs and contribute to significant mass transfer from the upwelling asthenospheric mantle to the arc crust.