Caractéristiques des diverses inclusions associées aux serpentinites de la nappe des Gets (Haute-Savoie, France)

The "Nappe des Gets" is the highest of a series of tectonic units, composed mainly of allochthonous flysch of Upper Cretaceous age, which together form the "Nappe supérieure des Préalpes". The essential characteristics of the Nappe des Gets are: the association of different kinds of flysch, ranging in age from the Albian to, at least, the Turonian; the occurrence of olistoliths within some of the flysch; the anchimetamorphic character of some of the flysch. The olistoliths are composed mainly of ophiolites and of some granitic rocks. The ophiolitic lenses are interpreted as relics of a highly dismembered ophiolitic complex, probably within the Piemontaise palaeogeographic zone. Serpentinites, gabbros (not very abundant), diabases, ophiolitic breccias, pillow basalts and associated rocks have been recognized. The serpentinites, usually brecciated and "ophicalcitized", often contain various kinds of inclusions, e.g. blocks of gabbros, diabases, ophiolitic breccias and ophispherites. Some of these inclusions have been altered by metasomatic processes. Such phenomena are best developed in the ophispherites. This alteration, the result of some kind of rodingitization, shows characteristics which are unique to the Col des Gets region: a) The formation and growth of new calc-silicates is not of major importance; many inclusions show a core which is still quite fresh, b) Chloritization is often the only phenomenon producing the characteristic zonation of the ophispherites. At the Col des Gets area, the ophispherites are usually derived from hypabyssal rocks; an almost complete range from gabbros to fine-grained diabases is present. There are also a few small albitic inclusions. The fresh cores of the basic ophispherites show a sequence of original magmatic parageneses ranging from rare pyroxene-bearing rocks to rocks which contain basaltic hornblende (kaersutite) as the only mafic mineral. The dominant plagioclase is albitic but relics of an even earlier basic plagioclase (up to An₅₀) are found. If present the calcic minerals are: clinozoisite, rarely zoisite, prehnite and pumpellyite; garnet is missing. Two distinct episodes are proposed for the formation of the calcsilicates and the chloritization of the ophispherites. The freshness of some minerals of these inclusions, in particular the kaersutite, has made K-Ar dating possible. Results suggest that these rocks probably crystallized during Middle Jurassic times