CHEMICAL CHARACTERIZATION OF METABASITES FRONT THE TURTMANN VALLEY (VALAIS, SWITZERLAND) - IMPLICATIONS FOR THEIR PROTOLITHS AND GEOTECTONIC ORIGIN

Within the polymetamorphic basement of the Penninic Siviez-Mischabel n appe, metabasites are widespread occurring as small-scale lenses and l ayers that locally preserve remnants of a former metamorphic HP event. Metabasites of the eclogite localities (Adlerflue, Minugrat) in the u pper Turtmann valley were investigated by XRF and MAA means. Sampling rocks of different grade of metamorphic overprint enabled to estimate element mobility during amphibolite and Alpine greenschist facies over print. 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 a re basin. Despite repeated recrystallization, REE elements behaved imm obile in the cores of the lenses and layers independent of metamorphic grade, but were enriched at their periphery due to restricted mobilit y 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 Bts well into the situation of the European crust durin g the Cambro-Ordovician. Therefore, an early Paleozoic age for the met abasites is likely. The proposed model also favours the metabasites as a potential element source for the nearby Co-Ni ore deposits in the S iviez-Mischabel nappe.