Magma ascent mechanisms

We assume that the gabbro is still in its original orientation and was not tilted during subsequent tectonic events based on the following arguments: (1) the Campo unit record a weak Alpine overprint without evidence of major structures that may accommodate the tilting, (2) the top of the Sondalo gabbro and Campo unit is cross-cut by a sub-horizontal Jurassic shear zone over more than 20 km, (3) P–T conditions are similar all along the shear zone and apparently around the pluton, and finally (4) the Campo unit crops out continuously over more than 40 km with always similar regional P–T conditions (see chapter III). The Sondalo gabbro documents essentially sub-vertical magmatic foliations with sub-vertical magmatic lineations pointing to a vertical magmatic flow. Using structural, metamorphic and geochronological data, we propose a three-stage model for mafic magma ascent in the middle crust.

7.3.1 Stage 1: magma flow along host-rock anisotropy

The Sondalo gabbro was emplaced during the Permian in the Campo unit. This unit is characterized by a well-developed, pre-intrusion, regional Sc2 fabric steeply dipping either to the northeast or southwest (Fig. IV-2a). During its intrusion, the gabbro incorporated numerous

134 Formation et exhumation des granulites permiennes

xenoliths of the host-rock. The present-day shape preferred orientation of the xenoliths, which is parallel to the regional Sc2 fabric, indicates that most of them preserve their orientations and were not tilted during the intrusion (Fig. IV-3c). In the core of the pluton, the magmatic foliation is parallel to the xenolith orientation, with a mostly vertical magnetic lineation.

Therefore, magmatic flow occurred vertically in between the metapelitic screens without a significant deformation of the host-rock at this stage. Locally, the magma flow may wrap around the xenoliths and eventually the margins of the pluton, but is probably unable to significantly deform the host-rock.

Sc3 Sc3Sc3 Sc2Sc2

Sc2

Fig. IV-18: Evolutionary model depicting the emplacement mechanism during the multi-stage intrusion of the Sondalo gabbro. (a,b) Fracture-controlled magma ascent and (c,d) en-masse ascent of the core.

Insets illustrate the P–T evolution of metasediments (see chapter III for details), map view synthetic model and microstructures of magmatic rocks and position of ferromagnetic minerals. See text for details.

135

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

The presence of pre-Sc3 andalusite, that are statically growing far from the intrusion, as well as pre-Sc3 leucosomes in the contact aureole indicates that HT metamorphism occurred before the subsequent Sc3 deformation. In contact with the fertile metapelites, mafic magma caused HT contact metamorphism grading from upper amphibolite conditions in the contact aureole to granulite facies conditions in the core of the pluton (chapter III). Granitic melt segregated from the migmatites contaminated the mafic magma, as indicated by the trace element composition of zircon, and the LREE enrichment of the magma with respect to a parental N-MORB liquid (Tribuzio et al., 1999). The highest metamorphic conditions are recorded in the core of the pluton, where Grt–Sil–Crd granulites equilibrated at 5.5 kbar/930°C, indicating that the presently exposed xenoliths in the core of the pluton were located at around 18 km depth during this intrusion stage. Conversely, the presence of andalusite in the host-rock and the contact aureole indicates a shallower depth of ~15 km (P < 4.5 kbar) during the intrusion. The emplacement of mafic magmas occurred around 289 ± 4 Ma. It is in agreement with the similar age obtained in two migmatite samples of the contact aureole (BPA 003-12c and d) dated at 289

± 4 Ma and 288 ± 5 Ma.

7.3.2 Stage 2: rise of the core of the pluton and material transfer in the aureole

The second intrusion stage is indicated by a change in the rheological behavior of the host-rock with respect to the magma. The steeply north-dipping magmatic foliation which originated during phase 1 is transposed into a sub-vertical to vertical magmatic foliation that follows the border of the pluton. This process is essentially documented in the western margin of the pluton, where the strike of this new fabric lies at high angle to that of the foliation in the CZ. The new magmatic foliation is concordant with the Sc3 foliation which is essentially present in the contact aureole and in few marginal xenoliths. This relation is interpreted as the deformation of the host rock during a second phase of the pluton emplacement. The pluton is still increasing in size, as indicated by the incorporation of a few folded and deformed xenoliths (e.g. in the westernmost part of the pluton, Fig. IV-2a). Eventually, the end of the intrusion may be marked by compaction in the rim of the pluton, explaining the planar AMS ellipsoid in the BZ.

Notably, the host Campo unit away from the intrusion is unaffected by the deformation (Fig. IV-18ab). In the chapter III, we showed that during Dc3, parts of the contact aureole are buried from ~4 kbar to 5.2 kbar and then exhumed back to 4 kbar (Fig. IV-18cd), whereas originally deeper rocks are exhumed from 6 kbar to the final emplacement depth of around 4 kbar (Fig. IV-18). Synchronously, internal xenoliths are exhumed and therefore transported from 5.5 kbar to the same final emplacement depth of 4 kbar. Consequently, the syn-emplacement

136 Formation et exhumation des granulites permiennes

Dc3 is associated to pervasive and ductile deformation associated to burial and exhumation of the host-rock in the contact aureole (Fig. IV-18cd). Alternatively, the requested burial and exhumation may be lowered by tectonic overpressure processes due to magmatic efforts or shearing in the contact aureole (e.g. Mancktelow, 2008). As xenoliths are exceptionally parallel, aligned along the host-rock foliation and does not seems to present different P–T evolutions, we suggest that all xenoliths were exhumed during the en-masse magma ascent episode (Fig.

IV-18cd). We suggest that shearing also occurred at the margin of the pluton and produced the magmatic foliation in the BZ while the core of the pluton remained only poorly affected.

An upper age limit for this process is given by the ca. 288 ± 5 Ma of migmatites showing evidence for supra-solidus Dc3 deformation (BPA 003-11c and d). A lower limit is 285 ± 2 Ma, given by diorite sample GM601 apparently cross-cutting the Sc3 deformation of the migmatites. Given the lack of evidence for sub-solidus deformation, we propose that the activity of Dc3 is restricted to supra-solidus conditions, and therefore unrelated to the activity of a shear zone as documented for some Permian lower crustal magmatic bodies (Zibra et al., 2012).

7.3.3 Stage 3: late-magmatic activity

The magmatic activity in the Sondalo gabbro continues after the final ascent with minor syn-magmatic dyking, as indicated by the 285 ± 6 Ma diorite and the 285 ± 2 Ma diorite cross-cutting the Sc3 foliation in the migmatitic contact aureole. Finally, a later and minor magmatic pulse seems to occur around 277 ± 3 Ma, but this age needs to be better constrained.

Alternatively, this age could correspond to the late crystallization of zircon in metamorphic rocks related to the main magmatic intrusion.