Tethyan vs. Cordilleran ophiolites: a reappraisal of distinctive tectono-magmatic features of supra-subduction complexes in relation to the subduction mode

Supra-subduction zone (SSZ) ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones. Moreover, owing to their structural characteristics and location in the overriding plate, these complexes are far better represented and preserved than Mid-Ocean-Ridge-Basalt (MORB) ophiolites in orogenic belts. In terms of their structure, tectonics, and magmatic features, SSZ ophiolites may be classified in two main types: (1) Tethyan complexes (such as those of the Albanide-Hellenide belt), which mostly consist of complete and extensive volcanic, dyke, plutonic, and mantle sections with prevalent Island Arc Tholeiitic (IAT) affinity and the significant presence of Boninites, obducted as relatively intact lithospheric slabs onto collisional continental margins; (2) Cordilleran complexes (including the circum-Caribbean ophiolites of Guatemala, Cuba, and Venezuela), mostly represented by dismembered sections of arc volcanic, plutonic, and subordinate mantle sequences with tholeiitic to Calc-Alkaline (IAC) magmatic affinity and acidic differentiates, commonly associated with metamorphic subduction complexes and tectonically emplaced onto or juxtaposed against the continental margin within polygenetic terranes. These two types appear to be related to significantly different subduction modes and intraoceanic plate dynamics whereby SSZ ophiolites were generated. The Tethyan complexes can best be compared with west Pacific-type subduction with accentuated steepening and retreat of the subducted stab, accompanied by progressive decoupling of the converging plates, intense mantle diapirism, and tensional events in the upper plate with generation of large IAT-sheeted dike complexes in open oceanic spreading systems. Continuous stab sinking and roll back allow increasing asthenospheric diapirism and interarc spreading from the arc axis to the forearc region with generation of boninites and Back Arc Basin Basalts (BABB) with intermediate MORB/IAT composition. By contrast, the genesis of the Cordilleran complexes requires a subduction mode characterized by a steady state regime, with moderate and constant dip of the subducted slab and limited extension in the backarc region. The magmatic evolution of these ophiolites from IAT to TAC and the significant presence of rhyodacite (and tonalite) differentiates coherently indicate a more mature stage of arc magmatism, as well as the occurrence of efficient differentiation processes developing under nearly closed-system conditions in independent magma chambers. Tethyan complexes are likely to be obducted as large and relatively intact slabs onto the continental margins through the interposition of metamorphic soles, which represent relics of the MORB lithosphere underplating the SSZ ophiolites. On the other hand, the common emplacement of Cordilleran complexes within polygenetic terranes appears to be controlled by prolonged accretionary mechanisms that trap, against the continental margin, parts of the arc structure, subduction complexes, melanges, and volcanoclastic products. (C) 2004 Elsevier B.V. All rights reserved.