Pre-Alpine metamorphism of the Internal zones of the Western Alps

Two types of basement are distinguished in the Internal Western Alps. (i) T he ancient basement is characterized by the following metamorphic facies se quence: eclogite (in the Ligurian Alps, followed by granulite +/- migmatite ) --> M-P, then lower-P amphibolite --> greenschist. On the basis of geolog ical and radiometric data from the Briancon zone the amphibolite facies is considered as Pan-African in age. However, it is also assigned a Variscan a ge in the Internal Penninic massifs, the Sesia zone and the Ligurian Alps. In the Briancon zone the amphibolite-facies imprint predates Ordovician gra nitoid intrusions and, in the Internal Penninic massifs (Gran Paradise, Mon te Rosa), it pre dates Variscan granitoid intrusions, which produced a cont act metamorphism. Permian felsic and mafic intrusions, also with a contact aureole, and Permian ages repeatedly found in micas are evidence of a wides pread thermal event of that age. This event ended with the Permian-Triassic intermediate and felsic volcanism commonly found in the Ligurian Alps. (ii ) The second type of pre-Alpine sequence,the so-called younger basement, wh ich occurs in part of the Briancon zone, possibly also in the Dora-Maira ma ssif. is assigned a lower Palaeozoic age and is lacking any identifiable pr e-Alpine metamorphic imprint. The basement units of the Western Alps are co nsidered to have originated from Gondwana and accreted to Eurasia during th e Alpine orogeny. However, according to some authors, they already accreted during the Variscan orogeny.