Eclogites from the Silvretta nappe (Switzerland): Geochemical constraints on the nature and geotectonic setting of their protoliths

Abstract

In the Austroalpine Silvretta nappe of central Europe eclogitic rocks of presumed Paleozoic metamorphic age have recently been found. Their chemical composition suggests that they were derived from evolved tholeiitic magmas which have undergone variable degrees of fractionation at shallow depths involving crystallization of plagioclase plus olivine and/or pyroxene. The igneous protoliths probably originated at a constructive plate margin. Although some chemical features closely resemble those of mid-ocean ridge basalts the composition of the metasedimentary country rocks suggests a tectonic setting close to a continental block, perhaps in a back-arc basin situation or incipient oceanic ridge.