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Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphically, show evidence for synmagmatic extension, and contain lavas with geochemical characteristics of arc-volcanic rocks. They are now inferred to have formed by hinge retreat in the forearc of nascent or reconfigured island arcs. Emplacement of these forearc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. A recent paper has challenged this interpretation. The authors assert that the “ophiolite conundrum” (seafloor spreading shown by dike complexes versus arc geochemistry) can be resolved by a model called “historical contingency,” which holds that most ophiolites form at mid-ocean ridges that tap upper-mantle sources previously modified by subduction. They support this model with examples of modern mid-ocean ridges where suprasubduction zone–like compositions have been detected (e.g., ridge-trench triple junctions).

The historical contingency model is flawed for several reasons: (1) the major- and trace-element compositions of magmatic rocks in suprasubduction-zone ophiolites strongly resemble rocks formed in primitive island-arc settings and exhibit distinct differences from rocks formed at mid-ocean-ridge spreading centers; (2) slab-influenced compositions reported from modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably with either modern subduction zones or suprasubduction-zone ophiolites; (3) crystallization sequences, hydrous minerals, miarolitic cavities, and reaction textures in suprasubduction-zone ophiolites imply crystallization from magmas with high water activities, rather than mid-ocean-ridge systems; (4) models of whole Earth convection, subduction recycling, and ocean-island basalt isotopic compositions ignore the fact that these components represent the residue of slab melting, not the low field strength element–enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; and (5) isotopic components indicative of mantle heterogeneities (related to subduction recycling) are observed in modern mid-ocean-ridge basalts (MORB), but, in contrast to the prediction of the historical contingency model, these basalts do not exhibit suprasubduction zone–like geochemistry. The formation of suprasubduction-zone ophiolites in the upper plate of subduction zones favors intact preservation either by obduction onto a passive continental margin, or by accretionary uplift above a subduction zone. Ophiolites characterized by lavas with MORB geochemistry are typically disrupted and found as fragments in accretionary complexes (e.g., Franciscan), in contrast to suprasubduction-zone ophiolites. This must result from the fact that oceanic crust is unlikely to be obducted for mechanical reasons, but it may be preserved where it is scraped off of the subducting slab.

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