Godard, G. and Martin, S. 2012. Evidence for Alpine Eclogite-facies Overprint in the Monte Rosa Massif (Alps). In: (Eds.) Michele Zucali, Maria Iole Spalla, and Guido Gosso, Multiscale structures and tectonic trajectories in active margins, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 41, paper 99.
Evidence for Alpine Eclogite-facies Overprint in the Monte Rosa Massif (Alps)
On the northern side of the Aosta valley (Italy), the Alpine collisional wedge is characterized, from bottom to top, by: (i) the Monte Rosa nappe, (ii) the eclogite/blueschist-facies Zermatt-Saas and Combin meta-ophiolites, and (iii) the upper Austroalpine units (i.e. Sesia-Lanzo, and the Dent Blanche klippe). These 3 complexes have been classically attributed to the pre-Alpine European continental margin (Monte Rosa), Piedmont ocean (meta-ophiolites), and Adria continental margin (Austroalpine), respectively (e.g. Dal Piaz et al., 2001).
The Monte Rosa nappe may be in turn subdivided into three main units: (i) Variscan high-grade gneisses; (ii) Upper Carboniferous and/or Lower Permian granite-granodiorite plutons; (iii) remnants of Permian-Mesozoic sedimentary cover and the composite Furgg zone (Bearth, 1956). The pre-granitic basement is composed of paragneiss, micaschists, and migmatite (see Dal Piaz, 2001), which have undergone high-T low-P metamorphism during the Variscan orogeny (e.g. Engi et al., 2001). They were sharply intruded by Upper Carboniferous granodiorites and Permian granites (Hunziker, 1970; Liati et al., 2001; Scherrer et al., 2001). The whole Variscan basement was deformed and metamorphosed during Alpine orogeny.
On the southern side of the Monte Rosa Massif (e.g. Rifugio Città di Mantova, Pian di Veraz, Brusson), the migmatitic paragneisses, as well as the orthogneisses that derived from the Variscan granites, show petrological evidence for two distinct metamorphic stages. An early high-T low-P paragenesis, coincident with migmatisation in the paragneiss, consists of biotite + quartz + plagioclase + garnet ± cordierite ± K-feldspar + ilmenite. During a second stage, the rocks underwent a high-P metamorphism that is indicated by several metamorphic reactions:
Biotite + plagioclase = garnet + phengite + quartz + rutile [micro-coronas];
Biotite + K-feldspar = phengite + quartz + rutile [micro-coronas];
Biotite = garnet + phengite + rutile [pseudomorphs];
Ilmenite + plagioclase = garnet + rutile + quartz [coronas];
Cordierite = garnet + micas + kyanite ± quartz [pseudomorphs];
Plagioclase = albite [possibly after jadeite] (+ phengite + zoisite + kyanite) [pseudomorphs];
Monazite + plagioclase = apatite + allanite [micro-coronas].
These reactions, which produced garnet, rutile, and phengite at the expense of biotite, ilmenite, K-feldspar and plagioclase, are typical of a high-P eclogite-facies metamorphism.
These two stages belong to the Variscan orogeny (migmatisation and granitic intrusions) and the Alpine orogeny (eclogite-facies coronas and pseudomorphs). Their P-T conditions have been estimated to be T ≈ 700°C and P ≈ 6 kbar, and T = 600-700°C and P > 20 kbar, respectively. This metamorphic history seems characteristic of the whole Monte Rosa basement, since evidence for eclogite-facies metamorphism has also been mentioned elsewhere in the Massif (Chopin & Monié, 1984; Dal Piaz & Lombardo, 1986; Le Bayon et al., 2001). Very similar rocks are also well known at Monte Mucrone (Alps, Sezia-Lanzo) and have been observed in the Variscides (see Godard, 2009).
The Monte Rosa nappe would represent a slice of the Variscan European continental crust that underwent subduction and subsequent eclogite-facies metamorphism during Alpine convergence. In this part of the Alps, not only the meta-ophiolites and the upper Austroalpine nappes show an eclogite-facies metamorphic imprint, but the Monte Rosa basement as well, suggesting that most of the Alpine collisional wedge underwent subduction at an early stage of the Alpine orogeny.
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