Interaction between mid-ocean ridge and subduction magmatism in Albanian ophiolites

Albanian ophiolites are represented by two different coeval belts, each displaying well-exposed, complete ophiolitic sequences that originated in the same oceanic basin and each showing distinct geochemical characteristics. The eastern belt is characterized by suprasubduction zone (SSZ) ophiolitic sequences, including island arc tholeiitic and boninitic volcanic series. The western belt, although composed mainly of mid-ocean ridge-type (MOR-type) ophiolites with high-Ti geochemical affinity, also exhibits alternating sequences showing distinct geochemical affinities referable to MOR- and SSZ-type volcanics. These volcanics can be geochemically subdivided into four groups: (1) group 1 basalts show high field strength element (HFSE) and rare earth element (REE) concentrations similar to those of ocean-floor basalts; (2) group 2 basalts, basaltic andesites, dacites, and rhyolites, characterized by HFSE and light REE depletion similar to those in many low-Ti volcanics from SSZ settings; (3) group 3 basalts exhibit geochemical features intermediate between groups 1 and 2 but also bear SSZ features, being characterized by HFSE depletion with respect to the N-MORBs; (4) group 4 boninitic dikes display very low-Ti contents and typically depleted, U-shaped REE patterns. These different magmatic groups are interpreted as having originated from fractional crystallization from different primary basalts that were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Consequently, group 1 basalts may derive from partial melting of a fertile MORB source, while group 3 basalts may derive from 10% partial melting of a mantle that previously experienced MORB extraction. Finally, the group 2 basalts and group 4 boninites may be derived from about 10% partial melting of a mantle peridotite previously depleted by primary melt extraction of group 1 and group 3 primary melts. To explain the coexistence of these geochemically different magma groups, we present a model based on the complexity of the magmatic processes that may take place during the initiation of subduction in proximity to an active MOR. This model implies that the initiation of subduction processes close to such a ridge leads to contemporaneous eruptions in a fore-arc setting of MORBs (group 1) generated from the extinguishing MOR and the initiation of group 3 basalts generated in the SSZ mantle wedge from a moderately depleted mantle source. The development of the subduction in a young, hot lithosphere caused the generation of island arc tholeiitic basalts (group 2) and boninites (group 4) from strongly depleted mantle peridotites in the early stages of subduction, soon after the generation of group 1 and group 3 basaltic rocks.