Ophiolite concept and its evolution

Abstract

The ophiolite concept, fi rst developed in Europe in the early nineteenth century, went through several phases of evolution. Early studies of ophiolites prior to the plate tectonic revolution emphasized the development of ophiolites as in situ intrusions within geosynclines. The genetic association of mantle peridotites with volcanic and plutonic rocks in ophiolites was not considered in these studies, and the emplacement of serpentinized ultramafi c rocks in orogenic belts remained a topic of debate regarding ophiolites among the North American geoscientists. Recognition of extensional sheeted dike complexes, the existence of a refractory mantle unit represented by peridotites with high-temperature deformation fabrics, fossil magma chambers in plutonic sequences, and the allochthonous nature of ophiolites by the mid 1960s was instrumental in the formulation of the ophiolite model and the ophiolite-ocean crust analogy within the framework of the new plate tectonic theory. This analogy was confi rmed at the fi rst Penrose Conference on ophiolites in 1972, whereby an ideal ophiolite sequence was defi ned to have a layer-cake pseudostratigraphy complete with a sheeted dike complex as a result of seafl oor spreading. Ophiolites were interpreted to have developed mainly at ancient mid-ocean ridges through this model. Geochemical studies of ophiolites challenged this view as early as the beginning of the 1970s and suggested the association of magma evolution with subduction zones. This paradigm shift in the evolving ophiolite concept led to the defi nition of suprasubduction zone ophiolites in the early 1980s. Systematic petrological and geochemical investigations of world ophiolites throughout the 1980s and 1990s demonstrated the signifi cance of subduction zone derived fl uids and melting history in development of ophiolitic magmas; forearc, embryonic arc, and back-arc settings in suprasubduction zones became the most widely accepted tectonic environments of origin. Major differences in their internal structure and stratigraphy, extreme variations in their chemical affi nities and mantle sources, and signifi cant changes in the mode and nature of their emplacement in orogenic belts indicate that ophiolites form in a variety of tectonic environments and that they do not need to have a certain internal stratigraphy to them as defi ned at the 1972 Penrose Conference. A new classifi cation scheme presented in this paper considers seven specifi c types of ophiolites, based on their inferred tectonic settings of igneous origin and emplacement mechanisms in different kinds of orogenic belts (i.e., collisional versus accretionary). Application of this new ophiolite classifi cation scheme may prove helpful in recognizing the Archean oceanic crust and in better understanding the crustal and mantle processes in Earth’s early history.