IDENTIFICATION OF THE MANTLE UNIT IN OPHIOLITES: A MAJOR STEP IN THE EVOLUTION OF THE OPHIOLITE CONCEPT

During ~150 years prior to1965, a number of purely magmatic models were considered to explain the ophiolitic associations. The basic idea was that the puzzling association of volcanic and plutonic rocks in ophiolites was a result of magmatic differentiation in a huge and unique volume of magma. Ophiolite complexes were thought in this interpretation as autochtonous bodies with respect to their surrounding rocks. Around 1970, several authors described a mantle unit in the ophiolite complexes that they had mapped in detail. This unit was characterized by the occurrence of structures displaying high-temperature ductile deformation, and by a residual refractory composition of its peridotites. These features were in contrast with those observed in stratigraphically higher rock types (gabbros, diabase, lavas), which were ascribed to a crustal sequence that was devoid of any evidence for HT plastic deformation. This major discovery was endorsed by the 1972 Penrose Conference on Ophiolites, whose participants introduced the modern definition of a typical ophiolite sequence and thus the ophiolite concept. The discovery of a mantle unit in ophiolites coincided with the emergence of the Plate Tectonics theory, which provided a global mechanism for seafloor spreading at mid-ocean ridges. It subsequently became clear that any ophiolite complex exposed in a folded mountain range should represent a piece of fossil oceanic lithosphere on-land. Many authors re-examined the contacts of ophiolites with country rocks and demonstrated that their basal contact is generally tectonic, commonly marked by the presence of metamorphic soles. This observation resulted in the interpretation that ophiolites are actually allochtonous bodies. At that time R.G. Coleman defined the term obduction as a mechanism of tectonic emplacement of oceanic lithosphere onto a continental margin. With the discovery of the mantle unit in ophiolites, the purely co-magmatic model was replaced by a syn-genetic model, in which the residual mantle unit is considered as a counterpart of the crustal rocks, which represent the products of partial melting of the mantle.