P−T−d evolution of the Campo unit

6.1.1 Regional evolution

The sample BPA 099-12a displays evidence of a medium-pressure Barrovian prograde path for the Campo unit (Fig. III-14a). These involve growth of garnet in Sc1 and of garnet and staurolite during Sc2, both in the presence of muscovite and biotite. Initiation of the prograde path is marked by garnet core chemistry that fits with modelled isopleths around 4 kbar/540°C (circle 1 in Fig. III-10bc) and the peak pressure is reached at 6 kbar/600°C as indicated by garnet internal rim chemistry (circle 2 in Fig. III-10bcd). Garnet external rim composition indicates diffusion at 5.6 kbar/650°C (circle 3 in Fig. III-10bc). Crystallization of sillimanite in pressure-shadows of garnet due to Dc2 deformation in some samples (Fig. III-6c) also attests of such P–T conditions. Since muscovite remains usually stable and only rare leucosomes are present in the Campo unit far from the intrusion, maximal temperature may be slightly higher than muscovite-out/liquid-in line that lies around 650°C. The Barrovian P–T loop is followed by a decompression path that is not recorded by textural relationship but that should occur before the onset of the second prograde evolution. This second prograde path consists of an isobaric heating constrained by the growth of andalusite porphyroblasts (blades up to 10 cm in length) and by diffusion in staurolite rim at P ≈ 4 kbar and T < 580°C as no sillimanite is observed (circle 4 in Fig. III-10d). Such prograde path questions the eclogite-facies conditions described in the literature (20 kbar/800°C, Gosso et al., 1995).

6.1.2 Evolution of metasediments in and around the pluton

In the contact aureole, the sample BPA 003-11a (Fig. III-14b) preserves evidence of the prograde evolution: (1) presence of garnet cores that could by identified by their compositional zoning (Fig. III-9b) and their textures indicating a growth in a pre-Sc3 structure (similar to Fig.

III-6e) where P–T conditions can hardly be constrained; (2) by the pseudomorphs of Sil–Spl–

Ilm±Crn after pre-Sc3 andalusite with Bt–Ilm inclusions (Figs III-5b & III-7ab). The presence of pre-Sc3 leucosomes indicates that before Dc3, rocks were already partially molten. This sample presents striking similarities with the sample BPA 099-12a lying far from the intrusion.

However, andalusite pseudomorph points to a heating initiation below the aluminosilicate triple point at 4.5 kbar/500°C. This 4.5 kbar/550°C point contains a significant error bar, as it may be influenced by the minerals involved (e.g. fibrolitic/prismatic sillimanite, Kerrick, 1990). The sample reaches 5.2 kbar/800°C where the matrix equilibrated during Dc3 (circle 1 in Fig.

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Chapitre III : Métamorphisme post-varisque dans les Alpes

11bcd) and where the garnet grew while incorporating sillimanite, biotite and ilmenite grains (similar to Fig. III-6f). Garnet external rim was partially re-equilibrated by late-diffusion at 4.8 kbar/770°C during retrogression (circle 2 in Fig. III-11bc).

Sample BPA 002-11d (Fig. III-14c) from the contact aureole presents few similarities with the other sample. The low Prp-core of garnet (Fig. III-9ce) with quartz and ilmenite inclusions (similar to Fig. III-6e) grew early during H₂O-saturated conditions not modelled by our pseudosection. Locally, this garnet present variations in Grs-content probably associated to an incorporation of calcium set free by the apatite consumption at the onset of partial melting (e.g. Indares et al., 2008). Prp-rich garnet rim with light oscillatory Grs zoning (Fig. III-9ce) and Sil–Bt–Ilm inclusions (similar to Fig. III-6ef) constrains conditions of 6 kbar/750°C where

P (kbar)

Regional metamorphism - micaschist BPA 099-12a Contact aureole - kinzigite BPA 003-11a

Xenolith - granulite BPA 018-11a Contact aureole - kinzigite BPA 002-11d

Fig. III-14: Sketches summarizing the inferred P–T path and associated crystallization/deformation relationships for (a) regional metamorphism, (b, c) samples in the contact aureole and (d) metapelitic xenoliths. See text for details.

86 Formation et exhumation des granulites permiennes

garnet diffused in presence of melt (circle 1 in Fig. III-12bcdef). Under these conditions, the matrix reequilibrated and produced pressure shadows around garnet associated to Sc3. Finally, garnet rim chemistry diffused at 5 kbar/725°C during the retrograde path (circle 2 in Fig. III-12bcde).

The granulite BPA 018-11a sampled in a metapelitic xenolith presents a higher T evolution (Fig. III-14d). Whereas it is not visible in this sample, garnet in xenoliths sometimes preserves inherited cores with textural features similar to the samples from the regional metamorphism (core of garnet in Fig. III-6e) and from the contact aureole, indicating a pre-HT prograde evolution. Most of the matrix minerals and inclusions of sillimanite, spinel and ilmenite in garnet do not present shape-preferred orientations or pressure shadows indicating that matrix equilibration and garnet growth occurred under static conditions. P–T conditions are constrained to 5.5 kbar/930°C (circle in Fig. III-13bce) by a matrix assemblage and a garnet flat compositional profile (Fig. III-9d) produced by diffusion. Such conditions are in agreement with stability of Bt-free assemblages during partial melting experiments (Patiño Douce &

Johnston, 1991; Vielzeuf & Montel, 1994). Dc3 deformation reorienting Sil–Spl grains remains weak and is limited to Fc3 microfolds of Sil–Spl grains (Fig. III-7e). K-feldspar that should appear during cooling before melt crystallization is not visible probably because of the low amount predicted by the modelling (around 0.01 mol. %). Diffusion affected garnet outermost rim and is characterized by a Fe–Mg exchange, Ca and Mn remaining constant (Fig. III-9d).

This indicates a partial re-equilibration of garnet by diffusion during cooling associated to slight decompression below 4.5 kbar.

In some samples collected in xenoliths, biotite presents lamellar intergrowth with quartz and is interpreted as back-reactions with melt or aqueous fluids (Waters, 2001). Staurolite stable in HT assemblages is described up to 1050°C if XFe is low (high Mg content, Grevel et al., 2002; Sato et al., 2010) or could be stabilized in supra-solidus conditions by a high Zn content (e.g. Ganguly, 1972). In our case, low Zn and Mg content, spinel inclusions (Fig.

III-7f), occurring in biotite or Mg-chlorite pockets indicate that staurolite occurs late in the prograde sequence or is retrograde. Therefore, we interpret these St–pockets to be a product of back-reaction with melt (e.g. Ashworth, 1975; Kriegsman & Hensen, 1998). Retrogression for the two samples from the contact aureole and the sample from a xenolith in andalusite stability field points to decompression below 4.5 kbar. In the samples from the contact aureole, it is testified by the presence of a second generation of andalusite that grows statically (Fig.

III-7c), therefore after the end of Dc3 deformation. Dc3 deformation seems to be restricted to supra-solidus conditions, as quartz presents no recrystallization and occasionally only a weak undulose extinction.

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