The 500 Ma-old Thyon metagranite: A new A-type granite occurrence in the western Penninic Alps (Wallis, Switzerland)

The Thyon metagranite is located in the frontal part of the Siviez-Mischabel Nappe, in the western Penninic Alps. It is intrusive in a polymetamorphic banded volcanic complex as leucocratic concordant sills with pseudoaplitic rims. A distinct metamorphic schistosity is defined by dark-green Fe-rich biotite. Abundant mesoperthites, chess-board albite and low microcline are presumably related to magmatic stages and/or greenschist-facies metamorphic retrogression. Major, trace element and REE geochemistry, zircon typology, Y and Nb-bearing accessory minerals such as fergusonite and euxenite, all point to a metaluminous to peraluminous alkaline A-type granite. High-precision U-Pb zircon dating yielded a sub-concordant age of 500 +3/-4 Ma. The Thyon metagranite is the third record of a Cambro-Ordovician alkaline magmatic activity in the Alps. As A-type granitic magmatism is common in post-orogenic to anorogenic extensional tectonic regime, the Thyon intrusion could mark the transition between the Cadomian and the Caledonian orogenies.