INITIATION AND EVOLUTION OF INTRA-OCEANIC SUBDUCTION IN THE URALIAN OCEAN: GEOCHEMICAL CONSTRAINTS FROM DEVONIAN IGNEOUS ROCKS OF THE SOUTHERN URALIDES (RUSSIA)

During Devonian times, an arc-continent collisional orogen was created in the Southern Uralides (Russia), giving rise to the association of: a forearc complex including lavas, dykes and shallow intrusive rocks with boninitic, arc tholeiitic and calc-alkaline affinities; a fully developed intraoceanic arc (Magnitogorsk Arc, MA); an accretionary complex developed over the subducting slab; a suture zone of the arc to the continental margin, the Main Uralian Fault. The latter is marked by obducted ophiolites and exhumed high-P metamorphic rocks of continental origin. The ophiolites include mantle lherzolites, mantle-crust transition sequences, incomplete plutonic sections, and normal MORB's capped with Ordovician cherts. The Early-Middle Devonian MA extrusive rocks include suites with boninitic, depleted island-arc tholeiitic, calc-alkaline, and transitional MORB-arc tholeiitic affinities. Late calc-alkaline to moderately alkaline dykes cut both the Ordovician basalts and the mantle ophiolites. Immobile incompatible trace elements (REE, Th, Nb and Ta) and Sr-Nd-Pb isotope geochemistry of the Devonian island arc-related rocks discriminate different source components. The MA magmatic suites and the dykes are derived from spatially and temporally diverse mantle sources with various enrichments in slab-derived fluid/melt components. In the dykes, Nb enrichment and high Nb/Yb ratios with moderate HFSE depletion may reflect either an enriched ocean-island basalt (OIB)-type component or slab melt addition to the mantle wedge. According to recent models for initiation of subduction, we propose that this enriched, OIB-type mantle component was located in a thickened, strongly depleted oceanic lithosphere, likely formed by the activity of a mantle plume. This oceanic plateau, buoyant with respect to the more fertile oceanic crust around it, triggered the initiation of subduction, starting the abundant boninitic magmatism in the MA, rapidly followed by tholeiitic and calc-alkaline magmatism.