s tructuraL evoLution

The area of interest, preserving the pre-Alpine structures encompasses the Campo unit W of the Sondalo gabbro and N of the Grosina unit (Figs III-1b & III-2a). Structural framework is described as follows: “D” stands for a deformation event responsible for the development of folds “F” and schistosity “S”, “c” refers to the Campo unit, “g” to the Grosina unit and “a” to alpine structures and numbering refers to the relative chronological sequence of deformations/

fabrics determined from field and microstructural observations

3.1 Regional Sc1 and Sc2 fabrics

In the Campo unit, a first macroscopically visible fabric is a NE-SW trending steep Sc1 foliation (Fig. III-2ac), preserved mainly in the north of the study area. Sc1 is marked by compositional layering, alternation of mica- and quartz-rich bands (Fig. III-4ab) and by preferential orientation of muscovite and biotite in micaschists. Few folded quartz segregations within the Sc1 foliation represent pre-Sc1 quartz veins (Fig. III-4a & III-5a).

Sc1 fabric is affected by open to isoclinal Fc2 folds with steeply plunging fold axis and steep NW-SE trending axial planes. The Fc2 folding leads to development of a Sc2 cleavage (Fig. III-4a) and in most places to almost complete transposition into a Sc2 foliation (Fig.

III-4b). Thus, the main structure in the Campo unit is the steep NW-SE trending Sc2 fabric (Figs III-2abc & III-3) defined by compositional layering, alternation of mica- and quartz-rich layers, alignment of elongated quartz segregations as well as probable rare leucosomes, and by preferential orientation of biotite and muscovite (Fig. III-4bc). The Sc2 fabric is crosscut by undeformed to weakly deformed mid-Permian pegmatites (Fig. III-4c).

Within the Sondalo gabbro, metapelitic septa are preferentially oriented parallel to the Sc2 fabric of the Campo unit (Fig. III-2b). At the outcrop scale, Sc2 within the septa is defined by compositional layering (Fig. III-4h), elongated quartz segregations (Fig. III-4g), and by shape-preferred orientation of biotite and sillimanite.

63

Chapitre III : Métamorphisme post-varisque dans les Alpes

3.2 Regional Sc2 fabric preserved in the pluton

The Sondalo gabbro displays locally a magmatic foliation marked by preferred orientation of pyroxene, amphibole and plagioclase, and in places by an alternation of pyroxene and amphibole rich layers with plagioclase rich layers. The magmatic foliation is parallel to the Sc2 foliation of the metapelitic septa and of the Campo unit (Fig. III-2bc).

3.3 Syn-intrusion Sc3 fabric

The Sc2 planar fabric is reworked in the contact aureole of the pluton by Dc3 deformation phase, in a zone ranging in thickness from meter- to few hundred meters and with intensity increasing towards the gabbro. In zones of weak Dc3 deformation, the Sc2 foliation is affected by plurimetric open Fc3 folds, with moderately plunging fold axis and axial planes dipping moderately away from the pluton centre (Fig. III-4d). In zones of strong Dc3 deformation, the Fc3 folds are tight to isoclinal with rare relicts of the Sc2 fabric (Fig. III-4e). Sc2 fabric is in places completely transposed into the Sc3 foliation parallel to the pluton margins and dipping moderately away from the center of the pluton (Fig. III-2bc). The Sc3 fabric is defined by shape preferred orientation of elongated quartz segregations and leucosomes and preferred orientation of biotite and sillimanite.

Dc3 is present in the few xenoliths close to the pluton boundary. It has a variable intensity from open microfolds (Figs III-4g & III-7e) to complete transposition of the previous foliation (Fig. III-4f). In both the contact aureole and in the xenoliths, leucosome are present in necks of boudins (inter-boudin partitions). Moreover, there is no solid-state recrystallization in the xenoliths and surrounding gabbroic and other magmatic rocks do not present sub-solidus deformation.

3 2 1 0 3

2 1 0

km 2 km

Eita shear zone

Grosina unit - orthogneiss & metapelite Campo unit - micaschist & paragneiss

Diorite Gabbro Ol-gabbro

Kinzigitic xenolith (Grt-Sil-Bt) Granulitic xenolith (Grt-Sil-Crd)

Migmatitic metasediments Sc1, interpretative Sc1 trace Sc2, interpretative Sc2 trace Sc3, interpretative Sc3 trace Da1 Thrust

Jurassic thinning structure BPA 018-11a

BPA 099-12a BPA 002-11d

BPA 003-11a

Location of samples used for petrology

Corn da Camp Val Valtellina

Grosina Sc1

Sc2 A Sc2 Sc3 B

Fig. III-3: Synthetic block along the transect A–B indicated on Fig. III-2a showing relationships between fabrics in the Campo unit around the Sondalo gabbro. Grosina unit in the upper part is modified from Mohn et al. (2012).

64 Formation et exhumation des granulites permiennes

Sc1

BPA 099 BPA 010 BPA 012

BPA 003 BPA 085

BPA 034 BPA 018

BPA 109

Fig. III-4: Field photographs of macroscopic structures. Regional structures: (a) micaschists with Sc1 fabric weakly affected by Fc2 folds and preserving pre-Sc1 quartz veins, location in Fig. III-5a; (b) paragneiss and micaschist folded by Fc2, micas are reoriented along Sc2 in the hinge zone; (c) steeply dipping Sc2 fabric crosscut by undeformed Permian pegmatite. Contact aureole: (d) Plurimetric Fc3 folds; (e) metatexite where melanosome preserves Sc2 fabric in Fc3 hinge. Metapelitic xenoliths: (f) melanosome boudins with leucosome-filled neck zones; (g) felsic granulite (Grt–Sil–Crd) with quartz lenses elongated in Sc2; (h) felsic granulite alternating with Qtz-rich layers along the Sc2 fabric, the yellowish color is due to abundant staurolite.

65

Chapitre III : Métamorphisme post-varisque dans les Alpes

3.4 Evidence for rift-related and Alpine deformations

In the Campo unit, a greenschist facies deformation overprints Carboniferous and Permian amphibolite facies structures approaching the rift-related shear zone separating the Campo and the Grosina units (Fig. III-2, Eita shear zone, see Mohn et al., 2012). It consists of sub-horizontal recumbent open folds affecting the Campo metapelites and Permian pegmatites, and localized shear-zones at the top of the Sondalo gabbro that transpose fabrics (described hereafter) only few meters below the contact with the Grosina unit.

Late Cretaceous Alpine thrusts (Da1) are localized at the top and the bottom of the Campo unit and consist of a few meters thick shear zone retrogressed to greenschist facies conditions. Locally, few normal faults (Da2) were described such as the “Mortirolo” fault of Late-Cretaceous age (Meier, 2003). During the Tertiary, Alpine deformation was responsible for large-scale gentle folds with ENE–SWS trending fold axis (Da3, Fig. III-2a) visible in the Val Grosina, but this deformation is not pervasive enough to disturb pre-Permian fabrics. The youngest deformation involves only few meters displacements along NW–SE oriented normal faults (Sistema della Piatta Grande, Corradini et al., 1973) associated to localized shear zones and pseudotachylites (see Meier, 2003 for description).