RE-OS, SM-ND, AND RARE-EARTH ELEMENT EVIDENCE FOR PROTEROZOIC OCEANICAND POSSIBLE SUBCONTINENTAL LITHOSPHERE IN TECTONIZED ULTRAMAFIC LENSES FROM THE SWISS ALPS

Thirteen serpentinized ultramafic lenses were studied to better unders tand their origin and constrain the pre-Ordovician history of the Cent ral European Variscides. The premetamorphic histories of these rocks w ere examined via the interpretation of rare earth element (REE) patter ns and the Sm-Nd and Re-Os isotopic systems. The geochemical evidence suggests that the ultramafic rocks represent at least two types of man tle material affected by a variety of metasomatic processes. One type of peridotite has REE compositions that are typical of harzburgitic or dunitic restites whereas the other type has REE abundances characteri stic of mixtures of restites and basaltic melts. In addition, some of the harzburgitic restites have light-REE enriched patterns that have b een interpreted as the result of post crystallization metasomatic effe cts in the mantle. The highly-depleted osmium isotopic compositions (m odern gamma(Os) -7.6 to -10.5) of three samples are indicative of a ma jor melt extraction event at ca. 2 Ga ago. This event Likely led to th e incorporation of the restites into stable subcontinental lithospheri c mantle (SCLM) which was subsequently isolated from the convecting ma ntle for at least 1 Ga. The Sm-Nd system records the breakup of this S CLM and its incorporation into the convecting mantle at about 0.9 Ga. The neodymium isotopic compositions of six samples are indistinguishab le from depleted suboceanic mantle at this time, and Likely represent remnants of oceanic lithosphere. Slices of this SCLM and oceanic mantl e were juxtaposed either during continent breakup or tectonically empl aced by the accretion of oceanic crust during the Ordovician. These re sults support the existence of a well-developed oceanic basin during t he late Proterozoic that was subsequently accreted onto an active marg in during the Caledonian orogeny of central Europe.