OPHIOLITE CONCEPT AND CONTROVERSY IN SEARCH OF ANCIENT ANALOGUES FOR MODERN OCEANIC LITHOSPHERE FOR >200 YEARS

The ophiolite concept, first introduced by Brongniart (1813) in reference to serpentinites in mélanges, has played a major role in the formulation, testing and establishment of hypotheses–theories in geosciences for over 200 years, and yet their definition, tectonic origin, geochemical affinities and emplacement mechanisms still remain controversial. Although the igneous architecture and extensional structures (sheeted dikes) in ophiolites lent strong support to the theories of seafloor spreading and plate tectonics in the early 1960s and then to their Penrose characterization in 1972 as an ancient analogue for modern oceanic crust, geochemical signatures of many ophiolites have subsequently shown that their crust is unlike typical, modern mid–ocean ridge (MOR) crust. This view, largely supported by the results of ocean drilling / dredging in convergent margin settings in the Western Pacific during the 1980s–1990s, led to the models of suprasubduction zone (SSZ) origin of ophiolites. This apparent conflict between the MOR vs. SSZ origin of ophiolites was later tagged as the “ophiolite conundrum” (Moores et al. 2000), which ascribed the slab and flux melting influence in their geochemical makeup to historical contingency of the earth’s mantle. However, this dichotomy is hinged mainly on a single template for ophiolites that does not exist, as the global ophiolite record shows significant variations in their internal structure, geochemistry and emplacement modes. The fossil oceanic crust preserved in ophiolites formed in different tectonic settings during the evolution of ocean basins, from the rift–drift and seafloor spreading stages to subduction initiation and terminal closure. Recent studies of peridotites and their chromitite deposits show inclusions of diamonds, UHP minerals and native elements in them, pointing to their origin in the mantle transition zone; such ophiolitic peridotites underwent multiple episodes of partial melting, depletion and refertilization during their ascent and petrogenesis. Future ophiolite research will shed further light on the nature and causes of mantle heterogeneities, mantle evolution patterns through time, earth’s deep recycling processes, and Carbon cycles, as ophiolites continue to facilitate transformative advances in many earth science disciplines.