The Albanian ophiolites are represented by two different coeval belts, each displaying well exposed, complete ophiolitic sequences which originated in the same oceanic basin, and each showing distinct geochemical characteristics. The eastern belt is characterized by supra-subduction zone (SSZ) ophiolitic sequences, including island arc tholeiitic and boninitic volcanic series. The western belt is mainly represented by mid ocean ridge-type (MOR-type) ophiolites with high-Ti geochemical affinity. Nonetheless, the western belt also exhibits volcanic sequences in which an alternation of rocks showing distinct geochemical affinities referable to MOR-type and SSZ-type volcanics can be observed. 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 volcanics include basalts, basaltic andesites, dacites and rhyolites, and are characterized by HFSE and light REE depletion similar to those observed in many low-Ti volcanics from SSZ settings; (3) Group 3 basalts are show geochemical features which are intermediate between Group 1 and 2. Nonetheless, they also bear SSZ features, being characterized by HFSE depletion with respect to the N-MORBs; (4) Group 4 is represented by boninitic dykes displaying 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 which 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 3 primary melts. In order 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 a subduction in the proximity of an active mid-ocean ridge. This model implies that the initiation of subduction processes close to an active mid-ocean ridge leads to contemporaneous eruptions in a fore-arc setting of MORBs (Group 1) generated from the extinguishing mid-ocean ridge, and 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 3 basaltic rocks.
Interaction between mid-ocean ridge and subduction magmatism in Albanian ophiolites
Saccani E.
2002
Abstract
The Albanian ophiolites are represented by two different coeval belts, each displaying well exposed, complete ophiolitic sequences which originated in the same oceanic basin, and each showing distinct geochemical characteristics. The eastern belt is characterized by supra-subduction zone (SSZ) ophiolitic sequences, including island arc tholeiitic and boninitic volcanic series. The western belt is mainly represented by mid ocean ridge-type (MOR-type) ophiolites with high-Ti geochemical affinity. Nonetheless, the western belt also exhibits volcanic sequences in which an alternation of rocks showing distinct geochemical affinities referable to MOR-type and SSZ-type volcanics can be observed. 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 volcanics include basalts, basaltic andesites, dacites and rhyolites, and are characterized by HFSE and light REE depletion similar to those observed in many low-Ti volcanics from SSZ settings; (3) Group 3 basalts are show geochemical features which are intermediate between Group 1 and 2. Nonetheless, they also bear SSZ features, being characterized by HFSE depletion with respect to the N-MORBs; (4) Group 4 is represented by boninitic dykes displaying 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 which 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 3 primary melts. In order 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 a subduction in the proximity of an active mid-ocean ridge. This model implies that the initiation of subduction processes close to an active mid-ocean ridge leads to contemporaneous eruptions in a fore-arc setting of MORBs (Group 1) generated from the extinguishing mid-ocean ridge, and 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 3 basaltic rocks.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
