Classification, Characteristics, and Origin of Ophiolites

Tholeiitic (TH) series rocks occur in almost all types of tectonic settings, whereas calc-alkalic (CA) series rocks occur characteristically in island arcs and continental regions. Abyssal tholeiites and associated gabbros in mid-oceanic ridges show much narrower ranges of $$SiO_{2}$$ contents and FeO* (total iron as FeO)/MgO ratios than volcanic rocks in oceanic islands and island arcs. Abyssal tholeiites and oceanic island volcanics tend to show higher $$TiO_{2}$$ contents than island arc volcanics. These are among the diagnostic features of volcanic rock series. In terms of volcanic rock series, the ophiolitic complexes may be classified into a number of distinct classes, three of which are discussed here. The ophiolitic complexes of Class I are characterized by the presence of volcanic rocks of both CA and TH series. A typical example is the Troodos massif in Cyprus, which was formed probably in an island arc. The ophiolitic complexes of Class II are characterized by the presence of TH series volcanics. Some of them were formed probably in island arcs and others in mid-oceanic ridges. The ophiolitic complexes of Class III are characterized by the presence of TH and alkalic series volcanics. Examples are ophiolites in high-pressure (glaucophanitic) metamorphic terranes. The volcanic rocks in such ophiolites show a marked chemical resemblance to the volcanics in some hot spot islands as well as to some volcanic rocks on stable continents. A few more classes may exist.