Ridge subduction, magmatism, and metallogenesis

Modern oceans contain large bathymetric highs(spreading oceanic ridges,aseismic ridges or oceanic plateaus and inactive arc ridges)that,in total,constitute more than 20–30%of the total area of the world’s ocean floor.These bathymetric highs may be subducted,and such processes are commonly referred to as ridge subduction.Such ridge subduction events are not only very common and important geodynamic processes in modern oceanic plate tectonics,they also play an important role in the generation of arc magmatism,material recycling,the growth and evolution of continental crust,the deformation and modification of the overlying plates,and metallogenesis at convergent plate boundaries.Therefore,these events have attracted widespread attention.The perpendicular or high-angle subduction of mid-ocean spreading ridges is commonly characterized by the occurrence of a slab window,and the formation of a distinctive adakite–high-Mg andesite–Nb-enriched basalt-oceanic island basalt(OIB)or a mid-oceanic ridge basalt(MORB)-type rock suite,and is closely associated with Au mineralization.Aseismic ridges or oceanic plateaus are traditionally considered to be difficult to subduct,to typically collide with arcs or continents or to induce flat subduction(low angle of less than 10°)due to the thickness of their underlying normal oceanic crust(>6–7 km)and high topography.However,the subduction of aseismic ridges and oceanic plateaus occurred on both the western and eastern sides of the Pacific Ocean during the Cenozoic.On the eastern side of the Pacific Ocean,aseismic ridges or oceanic plateaus are being subducted flatly or at low angles beneath South and Central American continents,which may cause a magmatic gap.But slab melting can occur and adakites,or an adakite–high-Mg andesite–adakitic andesite–Nb-enriched basalt suite may be formed during the slab rollback or tearing.Cu-Au mineralization is commonly associated with such flat subduction events.On the western side of the Pacific Ocean,however,aseismic ridges and oceanic plateaus are subducted at relatively high angles(>30°).These subduction processes can generate large scale eruptions of basalts,basaltic andesites and andesites,which may be derived from fractional crystallization of magmas originating from the subduction zone fluid-metasomatized mantle wedge.In addition,some inactive arc ridges are subducted beneath Southwest Japan,and these subduction processes are commonly associated with the production of basalts,high-Mg andesites and adakites and Au mineralization.Besides magmatism and Cu-Au mineralization,ridge subduction may also trigger subduction erosion in subduction zones.Future frontiers of research will include characterizing the spatial and temporal patterns of ridge subduction events,clarifying the associated geodynamic mechanisms,quantifying subduction zone material recycling,establishing the associated deep crustal and mantle events that generate or influence magmatism and Cu-Au mineralization,establishing criteria to recognize pre-Cenozoic ridge subduction,the onset of modernstyle plate tectonics and the growth mechanisms for Archean continental crust.