Moreau, Fabienne
[UCL]
Petrography, mineralogy and geochemistry of rodingites of the Piedmont ophiolite nappe, Western Alps (Italy). The focus of this research has been the estimation of the timing of the rodingitization process relative to the metamorphic evolution of the ophiolitic remnants in the Western Alps. Four rodingitic sites were studied: Perrères (Valtournanche, Val d'Aoste), Monte Avic (Val d'Aoste) and, in the Voltri Massif, Vara and La Carta. All of these occur in a high pressure – low temperature metamorphic context, retromorphosed to greenschist facies. For comparison, two samples from the mid-Atlantic fractures zones were examined. The petrographic study of the rodingites has revealed a primary paragenesis and a rodingitic paragenesis. The primary paragenesis comprises relict magmatic clinopyroxene (clinopyroxene 1) transformed into diopside, apatite and oxides (ilmenite and/or magnetite and/or chromite). The rodingitic paragenesis consists of garnet (grossular-alamandine and/or grossular-andradite), diopsidic clinopyroxene (clinopyroxene 2), chlorite (clinochlore), and epidote or vesuvianite. In the Voltri Massif, sphene, albite, amphiboles and carbonates are accessory minerals. The narrow rim of chloritoschist located between the rodingitic dyke and the surrounding serpentinite constitutes the only observable zonation. Epidote and vesuvianite represent the last phases of the rodingitic transformation process. A crystallisation – deformation study of the Perrères site, together with data on fluid inclusions in one sample from Vara, have shown that the garnet crystallized under greenschist facies conditions (i.e. 2 to 3 kbars and 360 to 400°C). Study of the rare earth elements (REE) and trace elements in rocks and individual minerals has led to the establishment of a model with three phases. The first phase is characterized by the clinopyroxene 1 which retained its REE-magmatic signature (isochemical); the second phase is represented by the garnet which is in equilibrium with clinopyroxene 2; while the third phase is marked by the appearance of epidote which seems to have grown at the expense of garnet. This study has also shown that the hypothesis which envisages an eclogitic protolith cannot be confirmed. Finally, the comparison between rodingites and the gabbroic samples collected by drilling in mid-Atlantic zones (Barling et al., 1997) suggests that the protoliths for the Perrères and Monte Avic rodingites were probably troctolitic cumulates and gabbros sensu stricto, while the Voltri Massif rodingites had an oxide-rich gabbro protolith. The initial liquids would have been of MORB-type , wich fit with the geochemical data of the alpine ophiolites.


Bibliographic reference |
Moreau, Fabienne. Pétrographie, minéralogie et géochimie des rodingites de la nappe ophiolitique piémontaise, Alpes Occidentales (Italie). Prom. : Verkaeren , Jean |
Permanent URL |
http://hdl.handle.net/2078.1/175672 |