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.