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.