Chemical characterization of metabasites from the Turtmann valley (Valais, Switzerland): Implications for their protoliths and geotectonic origin

Within the polymetamorphic basement of the Penninic Siviez-Mischabel nappe, metabasites are widespread occurring as small-scale lenses and layers that locally preserve remnants of a former metamorphic HP event. Metabasites of the eclogite localities (Adlerflüe, Minugrat) in the upper Turtmann valley were investigated by XRF and INAA means. Sampling rocks of different grade of metamorphic overprint enabled to estimate element mobility during amphibolite and Alpine greenschist facies overprint. Major and trace element contents are consistent with a primary tholeiitic basalt composition that originated from a subducting slab. Rising magma underwent fractional crystallization and/or assimilation of crustal material before and during extrusion within a narrow back arc basin. Despite repeated recrystallization. REE elements behaved immobile in the cores of the lenses and layers independent of metamorphic grade, but were enriched at their periphery due to restricted mobility along the compositional gradient between metabasites and host rock. Metamorphic compositional changes in K, Cr, and P can be explained by mineral breakdown and neoformation during retrogression. The proposed magmatic model fits well into the situation of the European crust during the Cambro-Ordovician. Therefore, an early Paleozoic age for the melabasites is likely. The proposed model also favours the melabasites as a potential element source for the nearby Co-Ni ore deposits in the Siviez-Mischabel nappe.