TI-CLINOHUMITE IN THE ECLOGITISED SERPENTINITES OF THE UPPER VALTOURNENCHE (ZERMATT-SAAS ZONE, ITALY): THE TECTONIC RECORD OF UHP IMPRINT?

The Zermatt-Saas Zone is the widely eclogites portion of the Thethyan oceanic suture in the Western European Alps buried during Alpine subduction. The presence of pre-Alpine relics shows that this oceanic crust was variously affected by oceanic metamorphism before subduction (Dal Piaz et al., 1980, Li et al., 2004).

Serpentinites with lenses and dykes of metagabbro and meta-rodingite displaying a metamorphic imprint in the eclogite facies, or greenschist facies, outcrop in upper Valtournenche. On the occasion of a detailed structural mapping three superposed foliations associated to different parageneses in all these rocks, followed by D₄ folding, has been recognised. In metabasites, S₁ and S₂ foliations are associated to high-pressure low-temperature parageneses, whereas S₃ is associated to a later re-equilibration in the greenschist facies. In serpentinites S₂ is the pervasive foliation, and it may be parrallel to bands rich in clinopyroxene₂ + amphibole₂. S₁ is recognised where S₂ is less pervasive: here relic Ti-clinohumite –olivine veins, up to 4 cm thick, boudinaged and folded in S₂ foliation, are found. Sometimes they have a white clinopyroxene rim toward serpentine.

Serpentinites mainly consist of serpentine with minor magnetite, but where S₁ and S₂ foliations are pervasive, metamorphic olivine is found, together with Ti-clinohumite and clinopyroxene.

The stable mineral assemblage associated with D1 is serpentine₁, clinopyroxene₁, opaques, titanite +/- amphibole₁, chlorite₁, olivine₁, Ti-Clinohumite₁, orthopyroxene, ilmenite. The assemblage stable with D₂ is serpentine₂, clinopyroxene₂, opaques, Ti-clinohumite₂, olivine₂, +/- chlorite₂, amphibole₂, titanite. Ilmenite and perovskite rim olivine and Ti-clinohumite. The assemblage associated with D₃ structures is serpentine₃, opaques, +/- chlorite₃ and amphibole₃.

Compositionally, serpentine associated to D₁ structures is more Mg-rich than that in D₂ structures. Olivine, clinopyroxene and chlorite do not have large chemical changes, though when associated to D₁ structures they may be more Mg-rich than when associated to D₂ structures. Ti-chumite₁ is slightly Fe-richer than Ti-clinohumite₂ and may have less Ti.

Veins consisting of olivine, pyroxene, Ti-clinohumite can predate or be contemporaneous to D₂ structures: post-D₂ veins never contain Ti-clinohumite, olivine and pyroxene. This conclusion, drawn on the evidences gathered through the detailed structural mapping, alows to constrain the timing of Ti-clinohumite growth to S₁ and S₂ foliations, contradicting the interpretation of Li et al., (2004) that interpret the veins as post D₃ or linked to ocean-metamorphism predating Alpine metamorphism. This result is coherent with what observed by Scambeluri & Rampone (1999) and Groppo & Compagnoni (2007) in other parts of the western Alps.

PT conditions estimated for S₁ and S₂ parageneses have been evaluated with preliminary calculations of a PT grid in the MASH system, using end-members (fosterite, talc, enstatite, antigorite, clinohumite, clinochlore, spinel and sudoite).

A PTtd path suggests that these rocks underwent pressures > 2.2 GPa for temperatures between 600° and 750°C, implying that they have been forged in a cold subduction zone characterised by a T/depth of ~6°C/Km. The late greenschist facies re-equilibration implies exhumation. It is characterised by a small T-decrease, indicating a thermal regime compatible with the thermal relaxation consequent to continental collision. Radiometric ages (Rubatto et al., 1998 and Lapen et al., 2003) suggest ages of 50-43 Ma for the P peak and ages of 36-40 Ma for the greenschist facies retrogression, suggesting exhumations rates between 0.6 and 2.7 cm/y for this part of the Zermatt-Saas unit.

References

Dal Piaz, G.V., Di Battistini, G., Gosso, G. & Venturelli, G. (1980). Arch. Sc. Geneve, 33, 161-179.

Groppo, C. & Compagnoni, R. (2007). Per. Mineral. 76, 127-153.

Lapen, T.J., Jhonson, C.M., Baumgartner, L.P., Mahlen, N.J., Beard, B.L. & Amato, J.M. (2003). Earth. Plan. Sci. Lett., 215, 57-72

Li, X-P., Rahn, M. & Bucher, K. (2004). J. Metam. Geol., 22, 158-177.

Rubatto, D., , Gebauer, D. & Fanning, M. (1998). Geochim. Cosmochim. Acta, 132, 269-287.

Scambelluri, M. & Rampone, E. (1999). Contrib. Min. Pet., 135, 1-17.