MAGMA GENERATION AND CRUSTAL ACCRETION IN SUPRA-SUBDUCTION ZONES AS DEDUCED FROM OPHIOLITES OF THE ALBANIDE-HELLENIDE SUBPELAGONIAN ZONE

Ophiolites of the Mirdita-Subpelagonian zone form a nearly continuous belt in the Albanide-Hellenide orogen and include mid-ocean ridge basalt (MORB) associations in the western Mirdita, as well as supra-subduction zone (SSZ) complexes, with prevalent island arc tholeiitic (IAT) and minor boninitic affinities, in the eastern part of the belt (e.g., Eastern Mirdita, Pindos, Vourinos, Othrys). In addition, basalts with geochemical features intermediate between MORB and IAT (MORB/IAT) are found in the central Mirdita and in the Pindos massif, where these basalts alternate with pure MORBs and are cut by boninitic dykes, as well as in the Othrys sub-ophiolitic mélange. Geochemical modelling carried out using real primary basalt-mantle residua pairs cropping out in the Mirdita-Subpelagonian zone indicate that the distinctive compositional characteristics of the mafic magmas parental to the different ophiolitic suites, as well as to their associated mantle residua can be accounted for by partial melting of mantle sources progressively depleted by melt extractions. Partial melting processes (10 - 20%) of lherzolitic sources generated pure MORBs, leaving as a mantle residuum cpx-poor lherzolites, which are very common in the Albanide-Hellenide ophiolites. The composition of IAT primitive basalts is compatible with about 15% fractional melting in a SSZ setting of these cpx-poor lherzolites. The mantle residua after IAT primary melt extraction correspond to depleted harzburgites found in many ophiolitic complexes of the eastern part of the Mirdita-Subpelagonian zone. About 10% water-assisted partial melting of the cpx-poor lherzolite source may reproduce the composition of MORB/IAT basalts basalts. The mantle residuum associated with these basalts correspond to harzburgites very depleted in light REE (LREE), Th, Ta, Nb, which are found in the Rhodiani massif (Vourinos). Boninites may have been derived either from high degree (about 30 %) partial melting of the same source or by comparatively lower degrees (10 – 20 %) of hydrated partial melting of more refractory harzburgitic sources. In both cases, a variable enrichment in LREE by subduction-derived fluids is required for explaining the relative LREE enrichment observed in most boninitic basalts. A generalized petrological model based on mass balance calculations between bulk rock and mineral compositions, indicate that most of the intrusive rocks (from ultramafic cumulates to gabbro-norites and plagiogranites) as well as sheeted dikes and volcanic rocks (from basalts to rhyodacites), forming the bulk crustal section of the SSZ ophiolites, may be accounted for by low-pressure fractional crystallization under partially open-system conditions from low-Ti picritic parental magmas very similar in composition to IAT picrites from Pacific intra-oceanic arcs. The most appropriate tectono-magmatic model for the generation of the SSZ Mirdita-Subpelagonian ophiolites implies low velocity plate-convergence of the intra-oceanic subduction and generation of a nascent arc with IAT affinity and progressive slab roll-back, mantle diapirism and extension from the arc axis to the forearc region, with generation of MORB/ IAT intermediate basalts and boninitic magmas.