Ultramafic Rocks in the Pre-Mesozoic Basement of the Central and External Western Alps

Abstract

Ultramafic rocks, usually accompanied by metagabbroic and or metabasaltic mafic rocks typically occur as small lenses of several to some tens of metres in thickness in many pre-Mesozoic basement units of the Central and external Western Alps. Some of these ultramafic-mafic rock suites can be followed along strike for several tens of kilometres. Mineral assemblages are mainly metamorphic with rare structural or mineralogical relics from the mantle or lower crustal stage. Depending on the grade of the Alpine metamorphism, these rocks are either serpentinites or olivine-schists with various hydrous or anhydrous Mg-Ca-Al-silicates, such as diopside, tremolite, chlorite, talc and opaque spinel. The marginal parts of these lenses are often hydrothermally altered to talc-amphibole- and carbonate-rich rocks. When the ultramafic rocks are accompanied by former gabbros and basalts, an ophiolite-type oceanic origin can be inferred. Some ultramafics are related to larger masses of metagabbros, metadiorites or amphibole-rich gneisses which are interpreted as former calc-alkaline intrusions in the lower crust. In the case of the larger masses of pure ultramafic rocks, a mantle-wedge tectonically emplaced in the crust along large-scale shear zones seems to be the most likely explanation. From the regional point of view the external massifs and the Penninic Adula nappe contain by far the most occurrences of mafic-ultramafic rocks. As yet, only little is known about the age of most of these series, but many of them have ages in excess of 450 Ma.