SUBDUCTION INITIATION RECORD IN FOREARC OPHIOLITES

The internal structure-stratigraphy and geochemical characteristics of forearc-SSZ ophiolites in different orogenic belts indicate a seafloor spreading origin in forearc-incipient arc settings during the initial stages of subduction. There is a well-developed magmatic stratigraphy in the extrusive sequences of these ophiolites from older MORB-like lavas at the bottom towards younger island arc tholeiite (IAT) and boninitic lavas in the upper parts. A similar progression of the lava chemistry also occurs in crosscutting dike swarms and sheeted dikes, indicating increased subduction influence in the evolution of ophiolitic magmas through time. Lherzolitic peridotites in structurally lower parts of the upper mantle sequences of these ophiolites represent the residue after MORB melt extraction. Harzburgite and harzburgite-dunite associations higher up in the mantle sequences and below the mafic-ultramafic cumulates (transitional Moho) are crosscut by networks of orthopyroxenite (opxt) veins, which include hydrous minerals (amphibole). The harzburgite-dunite-opxt suites characterize melt-residue relationships and melt migration patterns in the mantle wedge during the initial stages of subduction and incipient arc construction. Thus, most SSZ ophiolites display a lateral–vertical progression of melt evolution in their crustal and upper mantle components, racing different stages of subduction initiation-related magmatism, reminiscent of the forearc magmatism in some of the modern arc-trench rollback systems. The along-strike continuity for more than 1500 km of this well-documented chemostratigraphy and geochemical progression in different ophiolite belts is strong evidence for spontaneous subduction initiation followed by rapid slab rollback. We discuss in this talk the global forearc-SSZ ophiolite record and some of its key examples.