Reconstructing P-T paths from multi-stage garnets (Gran Paradiso nappe, Western Alps) - INSU - Institut national des sciences de l'Univers Accéder directement au contenu
Communication Dans Un Congrès Année : 2006

Reconstructing P-T paths from multi-stage garnets (Gran Paradiso nappe, Western Alps)

Pavel Pitra
Michel Ballevre
  • Fonction : Auteur
  • PersonId : 841742
Marcel Bohn
  • Fonction : Auteur
  • PersonId : 836392

Résumé

Most collision zones involve slices of continental crust that have been reworked during several, possibly unrelated, episodes. During the later metamorphic episodes, relics of the early history are progressively erased. However, traces of earlier metamorphic stages can be preserved either in low-strain domains, or within relic prekinematic phases in high-strain domains (e.g. garnets). Garnet-chloritoid-bearing micaschists from the Gran Paradiso massif (Western Alps) contain evidence of a polymetamorphic evolution. Detailed textural observations and detailed X-ray element maps reveal that two stages of garnet growth are present in the micaschists, interpreted as (i) relics of an early metamorphism of pre-Alpine age and (ii) newly-grown Alpine garnets, respectively. X-ray element maps for garnet 1 have revealed the occurrence of former fractures that cut across garnet 1, and were sealed during the growth of garnet 2. Furthermore, both generations of garnet preserve growth zoning. From THERMOCALC-based numerical modelling of mineral assemblages in P-T pseudosections, we infer that garnet 1 grew at increasing temperature and slightly increasing pressure, whereas garnet 2 grew at decreasing pressure and slightly increasing temperature. Estimated P-T conditions are about 620°C, 6 kbar for the peak of the pre-Alpine event, and of 490°C, 18-20 kbar for the pressure peak of the Alpine event. Modelling of the modal proportion and chemical composition of garnet (i) shows that the subsequent decompression (to 14-15 kbar at 550°C) must have been accompanied by moderate heating and (ii) does not support a stage of final temperature increase following decompressional cooling. This argues against a late thermal pulse associated with mantle delamination. Preservation of growth zoning in both generations of garnet and the limited amount of diffusive reequilibration at the boundary between the two garnets suggests that the rocks were subjected to fast burial and exhumation rates, consistent with data obtained from other internal Alpine units.
Fichier non déposé

Dates et versions

insu-00165514 , version 1 (26-07-2007)

Identifiants

  • HAL Id : insu-00165514 , version 1

Citer

Benjamin Le Bayon, Pavel Pitra, Michel Ballevre, Marcel Bohn. Reconstructing P-T paths from multi-stage garnets (Gran Paradiso nappe, Western Alps). Réunion des Sciences de la Terre 2006, Dec 2006, France. 1 p. ⟨insu-00165514⟩
211 Consultations
0 Téléchargements

Partager

Gmail Facebook X LinkedIn More