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Evidence for mantle heterogeneities in the Western Mediterranean: a statistical approach applied to geochemistry

AutorMelchiorre, Massimiliano; Vergés, Jaume ; Fernandez, Manel ; Coltorti, Massimo; Torné, Montserrat ; Casciello, Emilio
Fecha de publicación4-oct-2015
ResumenIn the last decades the Mediterranean has been the topic of many research investigations due to its complex geodynamic history. The results are reconstructions of plate kinematics, characterization of the volcanic activity that since Oligocene affected various areas from Spain to eastern Alps and from northern Africa to Germany and seismic images of the actual lithospheric domains beneath Europe and the Mediterranean. The combination of all this information gave rise to contrasting geodynamic models that can be broadly divided in three groups on the basis of the locations and dip of the subduction lithosphere: A) N-NW dipping slab extending from the Gibraltar arc to the Balearic promontory retreating in S-SE direction; B) NW dipping slab under the Balearic promontory, retreating in SE direction towards the Gibraltar strait C) two opposite-directed subductions separated by a transform fault subparellel to the Pierre Fallot Fault. The geological evolution of the Mediterranean is characterized by widespread volcanic activity, associated to subductive (orogenic) or intraplate (anorogenic) settings. Two huge databases of both types of volcanism ([1], [2]) have been merged and analyzed through factor analysis, in order to reduce the large number of geochemical parameters describing each sample (i.e. major and trace elements, isotopic compositions) through a smaller number of factors. The complete resulting database of ~14,000 geochemical analyses was lowered to ~600, choosing only those samples with a complete chemical profile. The factor analysis performed using the Principal Component Analysis method reduced the original 36 starting variables (i.e. 36 geochemical parameters) to seven factors accounting for ~84% of variance. Combining these factors in binary diagrams allow to clearly distinguish the anorogenic field from the orogenic one (except some overlaps due to the great number of samples used). Anorogenic samples usually fall in a narrow compositional range, while orogenic rocks are characterized by a greater variability and by alignment along different trends. These different trends account for large heterogeneities of the sub lithospheric mantle due to extensive recycling of geochemically different materials through time. This support the idea [3] that different reservoirs are responsible for the Mediterranean volcanism. The spatio-temporal evolution of the Western Mediterranean lamproites allows us to speculate a two-step evolution of the mantle beneath southern Spain. This evolution has been evaluated in light of the three models. We therefore propose that a type C model accounting for a double polarity subduction beneath the Western Mediterranean is the most feasible model to describe the geodynamic evolution of this area. [1] Lustrino and Wilson (2007). Earth-Science Reviews 81 (2007) 1¿65. [2] Lustrino et al. (2011). Earth-Science Reviews 104 (2011) 1¿40. [3] Conticelli et al. (2009). Lithos 107, 68-92.
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