Petrogenesis and tectono-magmatic significance of the Albanide-Hellenide ophiolites

The Mirdita-Subpelagonian ophiolites are part of a continuous ophiolitic belt extending from Dinarides to Albanides and Hellenides, and share geological, litho-stratigraphical, petrological, and metallogenic features. In the Albanide sector, the Mirdita ophiolites can be subdivided into two distinct belts: (1) an Eastern Belt composed of supra-subduction zone (SSZ) ophiolites with island arc tholeiitic (IAT) and boninitic affinities; and (2) a Western Belt mainly composed of mid-ocean ridge (MORB) ophiolites, locally associated with MORB/IAT intermediate basalts and boninitic dykes. In the Hellenide sector, the Subpelagonian ophiolites cannot be distinguished into two separate ophiolitic belts. In the Pindos Complex, the intrusives and the lowermost part of the volcanic sequence show MORB affinity, whereas its upper part displays MORB/IAT intermediate and boninitic characteristics. On the other hand, the Vourinos Complex is totally composed of SSZ ophiolites including a lower sequence (Krapa) with IAT affinity and an upper sequence (Asprokambo) with boninitic affinity. Petrological features of the different magmatic volcano-plutonic sequences suggest that they essentially originated by fractional crystallization from distinct parental magmas, which, in turn, were generated by partial melting of mantle sources variably depleted by previous melt extraction events. MORBs may have derived from partial melting of a relatively undepleted lherzolitic source, while MORB/IAT intermediate basalts generated by lower melting degrees of SSZ mantle source previously depleted by MORB extraction. Generation of IATs and boninites could, in turn, be accounted for by higher degree partial melting of progressively depleted mantle sources. Both IAT and boninitic sources were variably enriched in LFSE and LREE by subduction-derived fluids. The coexistence of these geochemically different magma types, in a restricted space/time interval, may be explained by various tectono-magmatic models. The favoured model for the genesis of the Mirdita-Subpelagonian ophiolites implies a formation of a IAT nascent arc by west-dipping intraoceanic subduction characterized by slab roll-back and intense asthenospheric mantle diapirism in the forearc region. This produced shallow partial melting of the sub-arc mantle with generation of boninitic magmas, and interarc tensional events with production of MORB/IAT intermediate basalts up to pure MORBs.