Jadeitite from the Monviso meta-ophiolite, western Alps: occurence and genesis.

A block is described, which is exposed in the antigorite serpentinite of Vallone Bulè, belonging to the Basal Serpentinite Unit of the Monviso massif (Piemonte Zone of calcschists with meta-ophiolites). The block consists of a quartz-jadeite rock core and a jadeitite rim, very similar to the rocks used by prehistoric men to make stone axeheads. In spite of their different bulk-rock compositions, both core and rim show the same trace and rare earth elements patterns, suggesting the same protolith. The quartz-jadeite rock exhibits a major, trace and rare earth elements composition consistent with that of oceanic plagiogranite, most likely a dyke cutting across upper mantle peridotites, later hydrated to serpentinites. Conversely, the jadeitite, which consists mainly of zoned jadeite crystals progressively enriched in the diopside component from core to rim, is significantly depleted in Si but enriched in Mg and Ca with respect to the quartz-jadeite rock. The trace and rare earth elements similarities and the ubiquitous presence of small zircons suggest that the jadeitite and the quartz-jadeite rock both derive from a plagiogranite; however, jadeitite would have undergone a metasomatic process involving a significant desilication and Mg- and Ca-enrichment, connected to the host peridotite serpentinization. The process, responsible for the transformation of the plagiogranite into a jadeitite, should have occurred during prograde Alpine high-pressure (eclogite-facies) metamorphism, since the first Na-pyroxene formed is jadeite, corroded and partly replaced during the metasomatic process by a progressively more omphacitic pyroxene. Because similar rocks - mostly jadeitites, but even their plagiogranite protoliths - are reported from other localities from the Western and Maritime Alps, it is likely that the raw materials of most jadeitites used to make stone axeheads, which are spread all over the Western Europe, have a similar origin and derive from the western Alps as long suggested.