Summary
Among the ophiolitic peridotites upwelled during the opening of the small Mesozoic Tethys, two groups have been recognized in the sector of the Western Alps: a) EO ultramafics (Mt. Civrari and Monviso) which are very depleted in easily fusible elements and followed a very high temperature (starting from 1300°C) decompressional trajectory; b) LA (Lanzo) and WO (Tour Real) ultramafics which are less depleted, followed decompression at slightly lower temperature (< 1200 °C) and might represent sections of shallower subcontinental mantle. The peridotites are associated with magmatic sequences showing affinities with N-MORB. These sequences derive from partial melting processes which left residua similar to the EO peridotites, but probably different from the LA-WO peridotites. These evidences demonstrate that small oceanic-type basins like the Mesozoic western Tethys are associated with MORB-type magmatism, but include mantle sections recording different steps in the evolution of the basin. Moreover these Tethys-like basins closed a few tens of m.y. after opening and their formation seems to be controlled by active tectonism and by local perturbations of large scale geodynamic processes.
Résumé
Parmi les péridotites tectonitiques remontées dans la Téthys Mésozoique lors de son ouverture on reconnait dans le secteur des Alpes occidentales: a) des ultramafites appauvries en éléments incompatibles qui ont suivi une trajectoire décompressionale à tres haute température (1300 °C) (M. Civrari et Monviso: EO); b) des ultramafites moins appauvries qui ont suivi une trajectoire à plus basse température (< 1200 °C) et qui pourraient représenter des témoins de manteau sous-continental (Lanzo: LA; Tour Real: WO). Ces peridotites sont associées à des séries magmatiques à affinité de N-MORB. Les péridotites EO peuvent représenter les résidus de la fusion partielle qui a produit les séries magmatiques de type N-MORB, cela n'étant pas probablement possible pour les péridotites LA et WO. Bien que les séries magmatiques des petits bassins océaniques de la Tethys Mésozoique occidentale montrent en général des affinités MORB, les péridotites témoignent de differentes étapes d'évolution des bassins. De plus ces petits bassins ont été fermés quelques m.a. après leur ouverture et leur évolution semble liée à une tectonique tres active et à des perturbations locales des phénomènes géodynamiques à grande échelle.
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This work is part of a C.N.R. (Centro di Studio sui Problemi dell'Orogeno dell Alpi Occidentali) programme.
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Pognante, U., Perotto, A., Salino, C. et al. The ophiolitic peridotites of the Western Alps: Record of the evolution of a small oceanic-type basin in the Mesozoic Tethys. TMPM Tschermaks Petr. Mitt. 35, 47–65 (1986). https://doi.org/10.1007/BF01081918
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DOI: https://doi.org/10.1007/BF01081918