GEOCHEMISTRY AND GEODYNAMIC SIGNIFICANCE OF NORTH PENNINIC OPHIOLITESFROM THE CENTRAL ALPS

In the Central Alps, underneath (i.e. west of) the main Alpine suture zone (Arosa Zone), ophiolitic rocks occur as isolated bodies and are c onsidered to represent remnants of the North Penninic or Valais ocean. The chemistry of metabasaltic rocks from these bodies displays charac teristics of transitional-type mid-ocean ridge basalts, slightly eleva ted in incompatible trace elements relative to N-type MORB. The rocks probably stem from a shallow asthenospheric source by ca. 15% partial melting. Ultramafic rocks associated with the metabasalts were formed from lherzolitic protoliths in a mid-ocean-ridge type environment. The association of peridotite, MORB-type basalt, and a pelagic sedimentar y sequence testifies of oceanic crust at least in parts of the North P enninic trough. The ocean floor presumably formed at a slow-spreading ridge in a transtensional basin. We propose a tight genetic connection of the mantle sources of the North and South Penninic oceans. The bas alts of the North Penninic ocean started to form when the South Pennin ic ocean already was in an advanced stage and displayed homogenization of its mantle source. The North Penninic ocean appears to have tapped this mantle source.