Geochemistry of eclogitised Fe-Ti-gabbros from different lithological settings (Aosta Valley ophiolites, Italian Western Alps). Protolith composition and eclogitic paragenesis

Eclogitised Fe-Ti metagabbros were collected from the Piemonte ophiolites of the Aosta valley (Italian Western Alps). These rocks are either associated with Mg-metagabbros or serpentinites, and show the same homogeneous metamorphic imprint. Petrographic observations on pseudomorphic replacement in undeformed pods and on mineral assemblages in tectonitic portions of the Fe-Ti metagabbros reveal the presence of three different high-pressure mineral parageneses. The first includes garnet, omphacite and rutile; the second consists of garnet, omphacite and glaucophane; and the third contains garnet, omphacite, tremolite and clinozoisite. Bulk rock chemical analyses show that all the studied samples derive from similar magmatic protoliths with tholeiitic affinity. However, differences in the amounts of major elements, namely TiO2, Na 2O, CaO and REE (particularly LREE) were found between the Fe-Ti metagabbros associated with Mgmetagabbros and those occurring with serpentinites. Different chemical compositions account for the three distinct eclogitic parageneses which developed in the two groups of metagabbros. We infer that both magmatic processes and rock alteration in an oceanic environment may have contributed towards producing these chemical differences. We suggest that Fe- Ti gabbros crystal lizing from similar magmatic protoliths, as a consequence of the different extent of alteration in the oceanic environment, developed distinct mineral assemblages during subduction-related Alpine highpressure metamorphism. This interpretation indicates that the early pre-Alpine history of the oceanic lithosphere in the Piemonte basin played an important role in determining the petrographic and geochemical features of its metamorphic products.