ReOs, SmNd, and rare earth element evidence for Proterozoic oceanic and possible subcontinental lithosphere in tectonized ultramafic lenses from the Swiss Alps

Abstract

Thirteen serpentinized ultramafic lenses were studied to better understand their origin and constrain the pre-Ordovician history of the Central European Variscides. The premetamorphic histories of these rocks were examined via the interpretation of rare earth element (REE) patterns and the SmNd and ReOs isotopic systems. The geochemical evidence suggests that the ultramafic rocks represent at least two types of mantle 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 characteristic of mixtures of restites and basaltic melts. In addition, some of the harzburgitic restites have light-REE enriched patterns that have been interpreted as the result of post crystallization metasomatic effects in the mantle. The highly-depleted osmium isotopic compositions (modern γ Os −7.6 to −10.5) of three samples are indicative of a major melt extraction event at ca. 2 Ga ago. This event likely led to the incorporation of the restites into stable subcontinental lithospheric mantle (SCLM) which was subsequently isolated from the convecting mantle for at least 1 Ga. The SmNd system records the breakup of this SCLM and its incorporation into the convecting mantle at about 0.9 Ga. The neodymium isotopic compositions of six samples are indistinguishable from depleted suboceanic mantle at this time, and likely represent remnants of oceanic lithosphere. Slices of this SCLM and oceanic mantle were juxtaposed either during continent breakup or tectonically emplaced by the accretion of oceanic crust during the Ordovician. These results support the existence of a well-developed oceanic basin during the late Proterozoic that was subsequently accreted onto an active margin during the Caledonian orogeny of central Europe.