Diversite des clinopyroxenites a grenat associees aux massifs ultramafiques orogeniques; eclogites, ariegites, griquaites et grospydites; une discussion

Abstract

Abstract Chemical composition of rocks belonging to the garnet clinopyroxenite clan (eclogites, griquaites, grospydites) are plotted in the Al <sub>2</sub> O <sub>3</sub> -FeO-MgO diagram. Compositions of eclogites from metamorphic series define a trend corresponding to liquids resulting from the low-pressure cotectic precipitation of ol+cpx+pl. These data support the igneous nature of the material and suggest that the original rocks experienced crystallization of the crust. Garnet clinopyroxenite inclusions from kimberlites and alkali basalts as well as garnet clinopyroxenite lenses from high temperature peridotite bodies define a different trend that can be interpreted as resulting from cpx+gt precipitation from a basic melt. The unusual position of these latter rocks justify that the term "griquaite" be applied to them. Corundum and/or kyanite bearing eclogites from metamorphic series as well as "grospydites" included in kimberlites or alkali basalts, or found as lenses in ultramafic bodies), plot toward the Al-rich apex of the Al-F-M diagram, but show different SiO <sub>2</sub> /Al <sub>2</sub> O <sub>3</sub> : the term "grospydite" appears fully justified to describe the latter, and distinguish them from the former (metamorphic) ones. The origin of grospydites is still in debate: they may be the products of high pressure crystallization of basic liquids. or of metasomatic crystallization of gabbros recycled within the mantle. The coexistence within the Beni Bousera massif in northern Morocco of griquaites and grospydites indicates the large heterogeneity of this upper mantle fragment. This heterogeneity can only be explained by a complex geodynamic evolution characterized by either 1) the succession of two distinct episodes of basic melt production; or 2) a cyclical mechanism implying the subduction in the mantle of a lithospheric fragment followed by the adiabatic ascension of this fragment towards the surface.