DEFORMATION-METAMORPHISM RELATIONSHIPS IN THE ECLOGITISED RODINGITES OF THE UPPER VALTOURNENCHE (ZERMATT-SAAS ZONE, ITALY)

The Zermatt-Saas Zone is a remnant of the Piedmont-Ligurian oceanic lithosphere, trapped in the suture zone of the Western Alps on the occasion of the Alpine subduction and collision. A major extensional fault (e.g Sartori & Thelin, 1987; Ballevre & Merle, 1993), whose recognition is grounded both on lithostratigraphic and metamorphic evidence (e.g. Bearth, 1967; Caby et alii, 1978; Dal Piaz, 1974; Kienast, 1983; Marthaler & Stampfli, 1989), puts it in contact with the Combin Zone. A structural analysis has been performed in a portion of the Zermatt-Saas Zone mainly consisting of mafic and ultramafic rocks of oceanic derivation and well known for its high- and ultrahigh-pressure mineral assemblages (e.g. Chinner & Dixon, 1973; Ernst & Dal Piaz, 1978; Reinecke, 1991; Xu-Ping et alii, 2004). Zermatt-Saas eclogites generally recorded high-pressure mineral assemblages formed by omphacite-garnet-chloritoid-talc-zoisite or omphacite-garnet-kyanite- clinozoisite ± talc (e.g. Barnicoat & Fry, 1986); eclogites are retrogressed into epidote amphibolites ± garnet.

The investigated rocks are mainly rodingites, associated with serpentinites, minor metagabbro and chlorite-schists. The mesostructural analysis allowed recognition of four groups of superposed ductile structures three of which are syn-metamorphic.Both D₁ and D₂ are correlated to the development of high-pressure parageneses. Mineral and textural relics older than these phases have been recognised and are correlated to the serpentinisation and rodingitisation processes ascribed to oceanic metamorphism.S₁ is a relic foliation recorded only in rodingite dykes and lenses, preserved in the millimetre thick microlithons of D₂ crenulation cleavage and as internal foliation of garnetI porphyroblasts; it is marked by pyroxene and amphibole SPO.

S₂ is the dominant fabric in rodingites and metagabbros; in rodingites it is underlined by alternating chlorite- and amphibole-layers and trails of small garnet₂.

In metagabbros pre-D₃ relics are garnet, omphacite, rutile and glaucophane + phengite and, successively, barroisitic amphibole, zoisite, Mg-chlorite, phengite and ilmenite.

D₃ structures are the most pervasive in serpentinites and gabbros, consist of a crenulation cleavage type foliation associated with open metric folds and are correlated to post-P peak greenschist facies retrogression. S₃ in rodingites is marked by Fe-chlorite, replacing zoisite. Both S₂ and S₃ affect the reaction rim underlying the serpentinite and rodingite boundaries, which consists of chloritic schist.

The last group of ductile structures is D₄ metric open folds with a sub-horizontal axial plane.

Structural mapping allowed the correlation of HP fabrics between metagabbros and rodingites, making possible to identify the HP assemblages in S₁ and S₂ fabrics in eclogitised rodingites: S₁- garnet₁, Mg-chlorite₁, epidote-clinozoisite₁, clynopiroxene₁, vesuvianite₁, titanite₁ + amphibole; S2 – Mg-chlorite₂, vesuvianite₂, clinopyroxene₂, epidote-clinozoisite₂, garnet₂ (in veins), titanite₂. These results highlight that vesuvianite forms together with chlorite, epidote, garnet, clinopyroxene HP assemblages and syn-D₂ recrystallization is associated with Al-increase.

Pre-D₃ PT conditions estimated in metagabbro of P ≤ 1.2 GPa for T ˜ 550°C suggests that these can represent the minimal pressure for vesuvianite recristallisation under HP conditions.

References

Ballevre M. & Merle O. (1993) -. Schweiz. Mineral. Petrogr. Mitt., 73, 205- 227.

Barnicoat A.C. & Fry N. (1986) -. Journ. Geol. Soc. London, 143, 607-618.

Bearth P. (1967) -. Geol. Karte Schweiz. N.F., 132, 1-130.

Caby R., Kienast J.R. & Saliot P. (1978) - Rev. Géogr. Phys. Géol. Dyn., 20, 307-322.

Chinner G.A. & Dixon J.E. (1973) - J. Petrol., 14, 185-202.

Dal Piaz G.V. (1974) - Schweiz. Mineral. Petrogr. Mitt., 54, 399- 424.

Ernst W.G. & Dal Piaz G.V. (1978) - Amer. Miner., 63, 621-640.

Kienast J.R. (1983) - Thèse d’Etat, Univ. Paris VI.

Marthaler M. & Stampfli G.M. (1989) - Schweiz. Mineral. Petrogr. Mitt., 69, 211-216.

Reinecke T. (1991) - Eur. J. Mineral., 10, 7-17.

Sartori M. & Thelin P. (1987) - Schweiz. Mineral. Petrogr. Mitt., 67, 229-256.

Xu-Ping L., Meinert R. & Bucher K. (2004) - Int. Geol. Rev., 46, 28-51.