MODELLO

Supra-subduction Zone (SSZ) complexes show distinctive structural and petrological features in Cordilleran ophiolites from the circum-Caribbean orogenic systems (in Guatemala, Cuba, Venezuela) and Tethyan ophiolites from the Albanide-Hellenide chains (in Albania and Greece). Tethyan complexes 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. Cordilleran complexes are mostly represented by dismembered sections of Island 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 ophiolitic types appear to be related to significantly different subduction modes and intra-oceanic plate dynamics whereby SSZ ophiolites were generated. The Tethyan complexes can be best explained by west Pacific-type subductions with accentuated steepening and retreat of the subducted slab, intense mantle diapirism and tensional events in the upper plate with generation of large IAT sheeted dike complexes in ?open? oceanic spreading systems. Continuous slab sinking and roll-back allow increasing asthenospheric diapirism from the arc axis to the forearc region, which may trigger: 1) shallow partial melting of the sub-arc mantle with generation of boninites and/or very low-Ti tholeiites, and 2) opening of a backarc basin with transitional MORB/IAT, up to pure MORB magmatism when mantle diapirs do not interfere anymore with the subduction zone. 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 complexes from IAT to IAC 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.