Late-Variscan tectono-metamorphic evolution

The end of the Variscan evolution in the Austroalpine domain is characterized by an extensional phase associated with exhumation and cooling of basement rocks (Fig. III-15b, e.g. at 310 Ma, Rb–Sr ages in Silvretta, Thöni, 1981). Evidence for exhumation between ca.

310 and ca. 290 Ma are preserved in the Bernina unit where alkaline intrusive rocks, emplaced at P < 3 kbar and 295 Ma, are crosscut by alkaline rhyolites dated at 288 Ma (Büchi, 1987;

Spillmann & Büchi, 1993; Von Quadt et al., 1994). Decompression of the Campo unit to P < 4.5 kbar occurs after the Variscan burial and exhumation, before the onset of HT regime associated with the emplacement of the Sondalo gabbro (samples BPA 099-12a and BPA 003-11a, Figs III-10, III-11 & III-14ab). But, no evidence of retrograde deformation could be found in the Campo unit. Therefore, we suggest that the Campo unit was tectonically unroofed during late-Carboniferous/Permian times, by a shear zone at the top of the unit.

From 290 to 270 Ma, the Permian post-Variscan extension is interpreted as characterized by a shallow lithosphere-asthenosphere boundary, responsible for the mantle partial melting and associated emplacement of mafic intrusion at all crustal level. In the Austroalpine domain mafic intrusions are emplaced in the lower crust (e.g. the Braccia gabbro, Hermann et al., 1997) and middle crust (e.g. the Sondalo gabbro, Braga et al., 2001; Tribuzio et al., 1999). In the case of the Campo unit, near-isobaric heating due to the intrusion (samples BPA 099-12a and BPA

88 Formation et exhumation des granulites permiennes

003-11a, Figs III-10, III-11 & III-14ab) occurred when the Campo present-day surface exposure was already lying at P < 4.5 kbar (Fig. III-15c), as attested by the static growth of andalusite and staurolite rim equilibration of samples BPA099-12a and BPA003-11a (labelled a and b on Fig. III-15). From a thermal point of view, it implies that rocks were re-equilibrated before the initiation of the heating. Similar P–T paths with a pre-Permian “orogenic” evolution followed by a Permian HT metamorphism are documented in the South Alpine realm (e.g. Barboza &

Bergantz, 2000; Benciolini et al., 2006). But contrary to Barboza & Bergantz (2000), who suggested that the Permian HT phase recorded in the basement occurs in continuity of the Variscan collapse, our data suggest that in some cases, crustal portions were (at least partially) thermally re-equilibrated before the onset of heating.

The intrusion of the Sondalo gabbro is responsible for HT metamorphism in the contact aureole and in metapelitic xenoliths before and during Dc3. A 2 kbar pressure gap occurs between samples far from the pluton (Fig. 14ab) and samples in the pluton (Fig.

III-0

Variscan crustal thicknening/peak-P/350-340Ma followed by peak-T/340-325Ma

D1-D2 Extensional collapse/decompression / 310-290Ma

Deep emplacement of the pluton / ~290Ma Static heating

So

So Final emplacement of the pluton / 288-282Ma D3

(a) (b)

First marine sediments/Cooling Sag-phase / 260Ma No pervasive deformation

No pervasive deformation in the Campo unit. Grosina unit thinning?

a in Carboniferous times; (b) extensional collapse of the orogenic crust associated with calc-alkaline to alkaline magmatism; (c) intrusion of the Sondalo gabbro in deep levels; (d) emplacement of the Feddoz and Braccia gabbros, final emplacement of the Sondalo gabbro in shallower crustal levels and associated contact metamorphism; and (e) post-tectonic cooling of the crust with formation of a sag-basin. See text for detailed explanations. Approximate position of the lithosphere-asthenosphere boundary is taken from the thermal model of Schuster & Stüwe (2008). So refers to Sondalo gabbro, Fe/

Br to Feddoz/Braccia gabbros.

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Chapitre III : Métamorphisme post-varisque dans les Alpes

14cd) at the initiation of the near-isobaric heating path. These two end-member samples are geographically separated by the contact aureole affected by the supra-solidus Dc3 deformation.

This points to a two stage intrusion. The mafic magma emplaced at ~18 km (~6 kbar) with few pipes rising up to ~12 km (~4 kbar) that are the Sondalo gabbro, but also other Permian intrusives in the Campo unit (Fig. III-15c) responsible for the development of pre-Sc3 static metamorphism. This was followed by a magma rise where deep xenoliths equilibrated at ~18 km are exhumed to ~12 km uring Dc3 shearing (Figs III-13, III-14d & III-15d). During this exhumation, samples from the contact aureole have a contrasted P–T evolution being either exhumed (BPA 002-11d, labelled c on Fig. III-15) or buried and latter exhumed (BPA 003-11a).

These P–T–d evolutions can give insights on the emplacement dynamics of the pluton, but needs to be constrained with more detailed characterization of magmatic fabrics.

The Permian HT-phase ended at ca. 260 Ma (Hermann & Rubatto, 2003). This event was followed by the deposition of thick Triassic succession (> 1km), passing from fluvial to shallow marine conditions. The deposition of these sediments is interpreted as related to thermal subsidence and possibly to tectonic subsidence in relation with the opening of the Neo-Tethys (Schuster & Stüwe, 2008). Geochronological arguments and field relations of the Austroalpine area in SE-Switzerland and N-Italy but also in the Alps in general, indicates that Permian HT rocks reach the surface only during Jurassic rifting (Mohn et al., 2012; Villa et al., 2000).

90 Formation et exhumation des granulites permiennes

7. c

The study of the metamorphic evolution of the Campo unit allowed us to constrain a polyphase Permo-Carboniferous P–T–d evolution that can be summarized as following:

- After a period of crustal thickening when rocks of the Campo unit reached 6 kbar/600°C resulting from Carboniferous Dc1 and Dc2 deformation stages, the Campo unit was exhumed before being intruded by Permian plutons.

- The mid-crustal gabbro intrusion in the Campo unit is responsible for host-rock partial melting and granulite facies metamorphism in metapelitic xenoliths in the pluton.

- Contrasted P–T paths from migmatites from the contact aureole indicate that the emplacement of the gabbro occurred in two steps. First, the gabbro intruded the deep Campo unit at 6 kbar. Second, an exhumation through Dc3 shearing in the migmatitic contact aureole placed the gabbro at 4kbar in the already shallower Campo unit.

- This HT-Permian evolution ends with formation of sag-basin and deposition of first marine sediments at ca. 260 Ma.

a

We acknowledge J.C. Corona, T. Theye and G. Morvan for guidance on microprobe from Amsterdam, Stuttgart and Strasbourg, respectively. K. Schulmann and E. Skrzypek are thanked for discussions.

CHAPITRE IV

92 Formation et exhumation des granulites permiennes

Le chapitre IV vise à contraindre les mécanismes ayant permis l’intrusion du gabbro de Sondalo dans l’unité de Campo, afin de comprendre comment se développe le magmatisme mafique dans des niveaux crustaux superficiels pour les magmas mafiques. Ainsi, ce travail est basé sur la cartographie des lithologies et les mesures des structures des roches magmatiques mais également de leur encaissant métasédimentaire. Ces campagnes de cartographie et d’échantillonnage ont eu lieu au cours des étés 2011, 2012 et 2013.

Cette section du manuscrit est axée sur trois parties :

– Les datations U–Pb de zircons extraits de roches magmatiques et métamorphiques, couplées aux analyses d’éléments en traces. La préparation et la séparation des minéraux ont été réalisées avec le concours de T. Mateeva, partiellement à la Vrije Universiteit d’Amsterdam, partiellement à l’Université de Strasbourg. Les mesures isotopiques U–Pb sur zircons ont été réalisées lors de séjours à l’Université de Lausanne avec l’aide de F. Galster, qui a par la suite complété les datations par des analyses des éléments en traces.

– La caractérisation des fabriques magmatiques et magnétiques. Cette section s’appuie sur des mesures de terrain, elles-mêmes couplées à une étude des structures magmatiques par Anisotropie de Susceptibilité Magnétique réalisée sur les roches magmatiques, en particulier sur celles dont les structures macroscopiques sont peu visibles. L’échantillonnage nécessaire a été effectué lors d’une campagne de forage en compagnie de G. Mohn, E. Skrzypek et T. Mateeva. La plupart des mesures ASM a été réalisée par T. Mateeva, ainsi que quelques-unes par A. Longeau (Université de Cergy-Pontoise).

– Une discussion des mécanismes d’intrusion du magma mafique dans l’unité de Campo, s’appuyant en partie sur un modèle thermique simple de l’intrusion.

Le contenu de ce chapitre fait l’objet d’un article en préparation, « Ages et mécanismes d’ascension et de mise en place dans la croûte continentale moyenne de magmas mafiques : l’exemple du complexe gabbroïque permien de Sondalo », qui sera soumis à la revue Tectonics.

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Chapitre IV : Mise en place d’un gabbro en croûte moyenne

TIMING AND MECHANISMS OF MAFIC