SUPRA-SUBDUCTION ZONE (SSZ) OPHIOLITES: IS THERE REALLY AN "OPHIOLITE CONUNDRUM"?

Supra-subduction zone (SSZ) ophiolites have been recognized in the geologic record for almost 30 years (Miyashiro, 1973). These ophiolites are essentially intact structurally and stratigraphically, show evidence for nearly 100% extension and contain lavas with geochemical characteristics of arc volcanic rocks. They are now inferred to have formed by hinge retreat in the fore-arc of nascent or reconfigured island arcs. Emplacement of these fore-arc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. In a recent paper, Moores et al (GSA Sp Paper 349) challenge this interpretation. These authors assert that the "ophiolite conundrum" (sea-floor spreading shown by dike complexes vs arc geochemistry) can be resolved by a model called "historical contingency" which holds that most ophiolites form at mid-ocean ridges which tap upper mantle sources previously modified by subduction. They support this model with examples of modern mid-ocean ridges where SSZ-like compositions have been detected (e.g. ridge-trench triple junctions).

The historical contingency model is flawed for several reasons: 1) slab-influenced compositions reported from modern ridge-trench triple junctions are subtle and/or do not compare favorably with either modern subduction zones or SSZ ophiolites; 2) crystallization sequences in SSZ ophiolites agree with subduction rather than MOR systems; 3) SSZ ophiolites include minerals (hornblende) and reaction textures that suggest crystallization from magmas with high water activities; 4) evolved lavas and volcaniclastics commonly overlie many SSZ ophiolites. Ophiolites characterized by lavas with MORB geochemistry are typically disrupted and found as fragments in accretionary complexes (e.g. Franciscan), in contrast to SSZ ophiolites. This must result from the fact that oceanic crust is unlikely to be obducted for mechanical reasons, but may be preserved where it is scrapped off of the subducting slab. Formation of SSZ ophiolites in the upper plate of subduction zones favors intact preservation. Finally, the historical contingency model fails because it requires us to believe that only ocean crust formed over previously modified mantle can be obducted, while true MORB crust is emplaced in accretionary complexes.