Greenshist altered metabasalts (ovardite) from Torre D' Ovarda (Val D' Ala, Graian Alps)

The term 'ovardite' was coined in 1873 and is still used in the Alpine literature with a variety of meaning. Examination of the 'ovardites' outcropping in Val d'Ala (Lanzo Valleys, Western Alps) shows that these are thin sequences on the roof of the ophiolitic sequence, on the border between volcanites and the sedimentary cover, consisting mostly of albite + chlorite (>60%), with subordinate amphibole, epidote, carbonate, sphene, quartz, opaques, rutile, apatite, white mica and biotite. These rocks, however, are different from the much more common 'prasinites', which consist of albite, actinolite, chlorite and epidote in roughly equivalent proportions, and are the greenschist Mid-Tertiary metamorphic product of basalts. Analysis of Val d'Ala ovardite showed marked Ca impoverishment and Al and Mg enrichment compared with its associated metabasalts (prasinites). The main trace elements (Zr, Nb, Y) point to the derivation of ovardite from materials with a MORB composition. In the light of the chemical and geological data, it is suggested that ovardite is a metamorphic product of basalts (probably tuff) altered by a palagonitic process, plus a debris contribution mostly in the form of carbonate.