OPHIOLITES: 4-D ARCHIVES OF EARTH'S MELT PRODUCTION AND RECYCLING PROCESSES

Ophiolites have been a continually evolving concept since 1813, when Brongniart introduced the term, and their recognition in the 1960’s as ancient oceanic lithosphere advanced the oceanic crust–ophiolite analogy and magma chamber models, lending support to the plate tectonics paradigm. By the early 1970’s ophiolite emplacement was considered as a first–order global tectonic event, leading to orogeny. The results of deep ocean drilling projects and structural–geochemical studies during the last three decades have shown, however, that ophiolites do not represent mid–ocean ridge (MOR) oceanic lithosphere, which largely gets subducted, except where some asperities in downgoing plates result in the preservation of its scraps in accretionary complexes. A vast majority of ophiolites formed in the extended upper plates of subduction zones (SSZ), whereby slab dehydration–fluid flux, fluid–rock interactions, corner flow, and slab rollback were significant processes in their melt evolution. Typical forearc ophiolites (FAO) contain lherzolites and harzburgites, showing increasing LREE enrichment with increasing whole-rock depletion, and textural–geochemical evidence for multiple episodes of melting, depletion and refertilization in a time–progressive evolution within <10 m.y. Their lavas range from N-MORB and medium–Ti basalts to IAT and boninitic rocks. Boninites are the products of high–degree (15–25%) partial melting of ultra–depleted residual peridotites during subduction initiation (SI) stages. Most FAO occur in collisional orogenic belts, where they make up part of continent–facing island arcs, emplaced on rift–drift sequences of continental margins. FAO igneous ages are predominantly Ordovician or Cretaceous, which mark the main ophiolite pulses in earth history. Chromitites–peridotites of FAO contain inclusions of microdiamonds and UHP minerals with P–T conditions of formation in highly reducing environments near mantle transition zone depths. Origin of microdiamonds is surface carbon recycled via subduction. Exhumation of microdiamonds, UHP minerals and their host chromitites to shallow mantle depths was facilitated by asthenospheric upwelling associated with slab rollback–induced channel flow in SI stages. FAO are the best 4-D archives of earth’s melt production and recycling processes.