Jadeite in the Gran Paradiso massif: the high-pressure metamorphic evolution of upper continental crust.

Christian Chopin (a), François Guillot (b) & Maria Di Rosa (a)
(a) Laboratoire de Géologie, Ecole normale supérieure – CNRS UMR8538, 24 rue Lhomond, 75005, Paris, France. Corresponding email: chopin@geologie.ens.fr Phone: +33 1 44 32 22 79. (b) Université de Lille – CNRS UMR8187, Laboratoire d'Océanologie et Géosciences, 59655 Villeneuve d'Ascq Cedex, France.

The formation and eventual preservation of high-pressure mineral assemblages in subducted continental crust are a long-standing issue with kinetic aspects and important geophysical implications in terms of buoyancy. The issue is especially acute in the internal crystalline massifs of the Western Alps, in which the dominant rock-type, metagranite, hardly bears evidence for high-pressure transformation whereas subordinate rock types clearly point to high or very high pressure conditions. The discovery of well-preserved jadeite-bearing rocks in orthogneiss of the western Gran Paradiso massif, reported here, revives the questioning and is the opportunity to describe uncommon mineral assemblages involving nepheline, preiswerkite, as well as a new trioctahedral mica.

Gran Paradiso massif, high-pressure metamorphism, jadeite, metagranite, metastability, subduction, reaction overstepping.