Ophiolites: Identification and tectonic significance in space and time

Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges, back arc basins, or forearcs during subduction initiation, and are key indicators of plate tectonics. Although it is widely agreed that ophiolites are remnants of oceanic crust and associated depleted mantle preserved in orogens, the recognition of ophiolites and their tectonic significance is still a topic of discussion and disagreement. We propose that ophiolites can be recognized in the geologic record by some combination of genetically related pillow basalt, layered gabbro, sheeted dykes, podiform chromite, harzburgite or/and dunite. Mafic igneous rocks have either ocean-ridge basalt or immature oceanic arc basalt chemical compositions. Using a scoring system of 1–11 for ophiolite confidence level, scores of ≥ 8 are considered confident, 6–8 probable, and < 6 questionable or unlikely ophiolites. Most ophiolites with scores ≥ 6 are < 900 Ma. The oldest confident ophiolite (score of 8) is the Zunhua ophiolite in eastern China at 2550 Ma, and the oldest well documented sheeted dykes occur in the Jormua and Purtuniq ophiolites at 2000–1950 Ma. Ophiolites do not become geographically widespread until after 900 Ma, and most ophiolites of all ages formed in a forearc (subduction initiation) tectonic setting. If ophiolite production requires plate tectonics, subduction must have begun at least locally by 2700 Ma but did not become widespread until after 2000 Ma. The abundance of ophiolites after 900 Ma may reflect better preservation of subduction-related ophiolites, or to an increasing global network of interconnected plates. Ophiolite frequency peaks in the geologic record partially reflect geographic regions where ophiolites have been extensively studied rather than monitoring the production rate of ophiolites, but the scarcity of > 900 Ma ophiolites is probably real.