Mélange Formation, Mantle-Wedge Diapirs And Subduction Zone Magmatism

Components derived from the subducting slab contribute to the source region of magmas produced at convergent plate margins. The characteristic range of compositions of these magmas is commonly attributed to three-component mixing in the source regions of these magmas: hydrous fluids derived from subducted altered oceanic crust and components derived from the thin sedimentary veneer are added to the depleted peridotite in the mantle wedge, which subsequently undergoes melting. However, the physical processes of transport and mixing of these components are largely enigmatic. In this presentation, we outline an integrated physico-chemical model of subduction zones: mélange formation at the slab-mantle interface is considered the physical mixing process that is responsible for the geochemical three-component pattern of arc magmas. Low-density mantle-wedge diapirs or plumes transport the well-mixed materials into the hot corner of the mantle wedge beneath arcs, where melt is produced by heating and decompression of the hydrous, low-density mélange plumes. Studies of exhumed subduction mélanges suggest that hybrid rocks with newly grown minerals concentrate, sequester and redistribute water and key trace elements. The strong petrologic and chemical contrast at the slab-mantle interface, produces these hybrid rock compositions by metasomatic reactions, diffusion and mechanical mixing: the Al-, Si- and alkali-rich slab that carries crustal isotopic signatures and trace-element abundances is juxtaposed with the Mg-rich ultramafic rocks of the harzburgitic mantle. Mechanical mixing of crustal and mantle rocks will propagate the formation of hybrid rocks, and fluxing by hydrous fluids derived from the dehydrating slab will enhance reactivity and lead to fluid saturation of the newly formed rocks. The rise of low-density plumes in the mantle wedge provides a mechanism to transport buoyant hybrid rocks from the slab-mantle interface toward the source region of arc magmas. Mélange rocks rising into the mantle wedge in 'wet' diapirs would be subjected to P-T conditions dramatically different from those at the slab surface. Partial melting of hybrid rocks may produce the large range of major and trace-element compositions found in modern island arc volcanic rocks.