PT EVOLUTION

All thermobarometric methods indicates that: (1) peak temperature during M2 was about 700°C (708° ± 47°C, using the database of Holland and Powell, 1990), for P values spanning from 2.8 to 4.4 kbar; (2) the evolution from M1 to M2 was related to a decrease in P associated to a T increase. The peak metamorphic conditions were reached close to the Bt-dehydration melting curve during or just after the D2 deformation (Fig. 10). Further evidence for high-T peak metamorphic conditions can be deduced on the basis of the following textural observations:

  1. In banded schists, interbedded between calc-silicate layers, hercynitic spinel, associated to Fe-Ti oxides, occurs within biotite-free regions, consisting of inclusion-free plagioclase, K-feldspar, and quartz. This textural evidence suggests that biotite was consumed and hercynite, Fe-Ti oxides, and melt were produced, possibly by a Bt-dehydration melting reaction.
  2. The presence of corundum bordered by K-feldspar in the high-grade schists and migmatitic gneisses is probably related to a muscovite-consuming reaction: Ms Crn + Kfs + H2O (Spear and Parrish, 1996), taking place at about 700°C for P conditions of 3-4 kbar (Fig. 10). This reaction occurred during the M2, and therefore it consumed previous Ms belonging to the M1 mineral assemblage.

The PT conditions of the high-grade gneisses during M2 are less well constrained. Partial melting producing the Crd-bearing leucosomes likely originated from a Bt-dehydration melting reaction: Bt + Als + Qtz + Vapor Crd + Melt (Fig. 10; Vielzeuf and Holloway, 1988), with cordierite occurring as a restitic phase. This reaction takes place at PT conditions consistent with the M2 metamorphism (Fig. 10).

In high-grade schists the PT conditions during the M3 are constrained by the presence of K-feldspar, close to the andalusite/sillimanite phase boundary (Fig. 10). Therefore, the M3 metamorphism took place in the retrograde portion of the PT path, at about 625°C and 2 kbar. This estimate is in agreement with the PT conditions calculated for the high-grade gneisses using thermobarometric methods. Further retrogression led to the instability of andalusite, which was replaced by muscovite.

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