The diversity of ophiolites

Abstract

Present studies reveal that ophiolites from the Alpine and Cordilleran orogenic belts may not represent deep abyssal oceanic crust but seem to have formed mainly in interarc or small ocean basin spreading centres. Abundant petrologic data on basalts from mid-ocean spreading centres (MORB) reveal a uniform and characteristic chemical nature. Current petrologic studies have shown an apparent scarcity of MORB within lavas of ophiolites from Phanerozoic orogenic belts. Peridotites from ophiolites reveal a complicated history of sub-solidus deformation and partial melting resulting from their diapiric rise into spreading centres and final incorporation into the oceanic crust. Structural studies reveal that ophiolites are emplaced either across passive continental margins or as basement fragments in accreted terraines. Attenuation of continental crust along passive margins in the formation of small ocean basin characterizes the tectonic setting for Tethyan ophiolites, whereas the ophiolites of Western North America are usually the oldest units in allochthonous tectonostratigraphic terraines and have formed in interarc spreading centres along the continental margins. Terraine accretion of these arc complexes preceded the continuous subduction of the Pacific and Farallon plates consisting of abyssal oceanic crust (MORB). Thus, ophiolite associations are mainly products of continental break-up or interarc spreading. Abyssal oceanic crust has been mainly consumed by subduction and only in extremely rare instances has it become accreted into continental crust.