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Comptes Rendus Géoscience
Volume 336, n° 9
pages 827-837 (juillet 2004)
Doi : 10.1016/j.crte.2003.12.024
Received : 10 June 2003 ;  accepted : 1 December 2003
Metastable staurolite-cordierite assemblage of the Bossòst dome: Late Variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees

Jochen E.  Mezger * ,  Cees W.  Passchier,  Jean-Luc  Régnier
Johannes Gutenberg-Universität, Institut für Geowissenschaften, Becherweg 21, 55099 Mainz, Germany 

*Corresponding author. Present address: Martin-Luther-Universität Halle-Wittenberg, Institut für Geologische Wissenschaften, Von-Seckendorff-Platz 3, 06120 Halle, Germany.

A kilometre-scale shear zone is recognized in the Cambro-Ordovician schist of the Bossòst dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism  , while coaxial conditions prevailed during later contact metamorphic   growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet-staurolite-andalusite-cordierite assemblages are significantly enriched in Mg and Mn over the garnet-staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet-staurolite assemblage preserves conditions during  , estimated by AFM diagrams and  -  pseudosections to be 5.5 kbar and 580°C, respectively. Pseudosections also indicate that staurolite is not a stable phase in cordierite-andalusite assemblages of  , suggesting polyphase metamorphism and decompression along a clockwise  -  path for the staurolite-cordierite-andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).


Les schistes cambro-ordoviciens du dôme de Bossòst, au sein de la zone axiale des Pyrénées Centrales, présentent une évolution tectonométamorphique polyphasée. Une première phase   s'exprime par une déformation non coaxiale permettant la nucléation de grenat et de staurotide syncinématique durant le métamorphisme régional. Lors de la phase   d'exhumation, l'andalousite et la cordiérite cristallisent en régime coaxial. Les analyses minéralogiques et sur roche totale montrent un enrichissement en Mg et Mn pour les échantillons à grenat-staurotide-andalousite-cordiérite, alors que ceux à grenat-staurotide-biotite, pauvres en Mg, ne permettent pas l'occurrence de cordiérite. L'analyse paragénétique des phases minéralogiques via deux pseudosections suggère une métastabilité pour la paragenèse staurotide-cordiérite et un chemin  -  horaire en décompression. Les paragenèses à staurotide-grenat-biotite, correspondant à la phase   du métamorphisme régional, cristalliseraient aux alentours de 5,5 kbar et 580°C. Cordiérite et andalousite apparaîtraient à basse pression durant la phase   d'exhumation initiée le long d'un détachement majeur au sein des schistes cambro-ordovicien. Pour citer cet article : J.E. Mezger et al., C. R. Geoscience 336 (2004).

Mots clés  : Variscan Pyrenees ; Bossòst dome ; staurolite ; cordierite ; mineral composition ; pseudosections ; polymetamorphism ; France.

Mots clés  : Pyrénées varisques ; dôme de Bossòst ; staurotide ; cordiérite ; composition minéralogique ; pseudosections ; métamorphisme polyphasé ; France.

© 2004  Académie des sciences@@#104156@@

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