THE ORIGIN OF GEOCHEMICAL COMPLEXITY IN OPHIOLITES

The presence of sheeted dyke complexes in ophiolites indicates formation in extensional environments, leading early workers to postulate formation at mid-ocean spreading ridges (e.g. Troodos). In the absence of detailed geochemical studies, ophiolitic lavas were assumed to have MORB compositions. Over the last 30 years increasingly detailed geochemical studies have shown that most ophiolites contain lavas, dykes, and plutonic rocks with clear subduction zone signatures, leading to the concept of suprasubduction zone origin of most ophiolites. Many ophiolites contain a range of igneous compositions from MORB or MORB-like lavas, to island arc tholeiites to boninites. The defining characteristic of an ophiolite is its emplacement on a passive continental margin, in an island arc or in an accretionary prism, which can only occur in convergent margins where buoyant material is emplaced beneath the oceanic lithosphere. Spreading in suprasubduction zones can occur in forearc, infant arc or backarc settings leading to the formation of sheeted dykes with arc-related compositions. MORB-like lavas can be generated in embryonic oceans (Red Sea or Ligurian-type) and in mature backarc basins, and pieces of MORB lithosphere can be trapped subsequently above intraoceanic subduction zones. Thus, ophiolites can contain a variety of chemical and crustal components formed in different tectonic regimes but assembled in suprasubduction zone environments prior to terminal collisions.