This study presents petrological and geochemical data on the mafic rocks of the metamorphic Mesozoic ophiolites of the west-central Alps: the Valais Zone, the Antrona Zone, the Zermatt­-Saas/Combin Zone and the region of Mt.Ziccher. Three groups of rock types have been discriminated based on bulk rock composition and field criteria: metabasalts, metamagnesiogabbros and metaferrogabbros.
The paleogeographic origin of the basaltic rocks was found to be transitional MORB according to the content of trace and rare earth elements.
From the first alpine metamorphic event (Cretaceous "Eoalpine" event), only a few relies have been preserved: omphacite-garnet in a symplectitic assemblage, barroisitic amphiboles and lawsonite pseudomorphs.
The major metamorphic event is Tertiary ("Mesoalpine") and exhibits a gradient from greenschist facies in the western part of the studied region to upper amphibolite facies in the eastern part. Minerals are systematically zoned, indicating a progressive evolution. The amphibole evolves from west to east from tremolite/actinolite to pargasite, and the plagioclase increases its anorthite content. The modal abundance of chlorite decreases towards the eastern part where clinopyroxene appears.
However, the three groups of rocks distinguished above each display a distinct evolution of minerai parageneses for the same metamorphic grade: the metamagnesiogabbros usually contain plagioclase richer in anorthite and amphibole poorer in aluminim and iron than the basalts.
In order to evaluate the increasing metamorphic grade, .corresponding to these minerai changes, some calibrations were applied: gamet-biotite geothermometry on associated pelitic schists, calcite­dolomite geothermometry on associated carbonate rocks, gamet-amphibole geothermometry on matie schists, an empirical geothermometer based on Ti content of the amphibole, and the reaction AB = ed + 4QTZ. Sorne isograds are mapped: oligoclase-, andesine- and anorthite-in for plagioclase; magnesio­hornblende-, tschermakitic hornblende- and pargasite-in for amphibole; chlorite-out, clinopyroxene-out as well as the isotherms between 500 and 700°C.