FROM BRONGNIART TO BOSTON: A HISTORY OF THE OPHIOLITE CONCEPT FROM 1813 TO THE PRESENT

Since Brongniart’s (1813) definition of ophiolite until the mid-1960’s, two contrasting views of ophiolites prevailed. Continental European geologists thought that there was a close association of serpentines, pillow lavas, and radiolarites ("Steinmann’s Trinity") which resulted from outpourings of mafic/ultramafic magma in the deep troughs of "eugeosynclines". English-speaking geologists argued that orogenic belt serpentines represented intrusions, either solid or magmatic, into "eugeosynclines", and they had no genetic connection to other parts of the "Trinity" . Studies of pseudostratiform ophiolitic sequences in Italy, northern Greece, and Papua in the mid 1960’s led to Hess’s (1965) acceptance of ophiolites as fragments of the oceanic crust. Studies in the late 1960’s on the Troodos complex, Cyprus, with its magnificantly exposed and preserved sheeted dike complex, (Gass, 1968, Moores and Vine, 1971) led to acceptance of ophiolites as fragments of oceanic crust formed at spreading centers. Ophiolites thus formed became a key ingredient in integration of land geology with "The New Global Tectonics". Since 1970, questions of the tectonic environment of ophiolite formation and emplacement have dominated the discussion. Chemical arguments that ophiolites formed mainly in a back-arc, or supra-subduction zone, setting have been highly popular, but new data from the oceans, the mantle, and the concept of "historical contingency" suggest that the tectonic environment cannot be determined by chemistry alone, but must also take into account the geology of each occurrence. The internal structure of many ophiolities can be compared with the structure of oceanic crust determined from ODP and other marine geological/geophysical studies. It is now possible to infer oceanic processes from ophiolites for times prior to 200 Ma. The tectonics of emplacement or "obduction" (Coleman, 1971) of ophiolites now seems clearly to be primarily the result of aborted subduction of continental marginal or island arc crust. The presence of ophiolitic rocks, included sheeted dikes, in the Proterozoic and Archean indicates sea floor spreading throughout Earth’s history. The Proterozoic may have begun when oceanic crust thinned to a subductable thickness.