Late Variscan volcanic activity is documented in the Late Carboniferous
Salvan-Dorenaz sedimentary basin and in the neighboring basement units
of the Aiguilles-Rouges and Mont-Blanc crystalline massifs (Western
Alps). Precise U/Pb isotopic dating, zircon morphology and geochemical
analyses indicate that volcanism occurred during short-lived pulses and
that coexisting crustal and mantle sources were involved in the
production of melts. Volcanic and subvolcanic products were emplaced
along major N-S to NNE-SSW transtensional fracture zones, similar to the
ones that governed intense basement exhumation and that favored the
formation and filling of the Late Carboniferous Salvan-Dorenaz
continental basin.
In the Aiguilles-Rouges massif, dacitic flows outcropping at the base of
the Salvan-Dorenaz basin erupted at 308 +/- 3 Ma; they represent the
surface equivalent of the nearby Vallorcine peraluminous granite and
associated rhyolitic dykes (311 +/- 17 Ma). In the Mont Blanc massif,
calc-alkaline rhyolitic dykes were emplaced simultaneously (307 +/- 2
Ma) at shallow crustal levels, but they derive from deeper magma sources
denoting enhanced mantellic activity.
Recently identified tuffs and volcaniclastic layers embedded at
different levels of the Salvan-Dorenaz stratigraphic record testify a
295 +3/-4 Ma old episode of highly explosive volcanism from distant
volcanic centers, possibly located in the Aar-Gotthard massifs (Central
Alps). Their zircon typology is highly heterogeneous. documenting
wall-rock contamination of the melts and/or admixture of crustal
sediments, whereas consistent subpopulations point to high-temperature
magmas of deep-seated origin and alkaline affinity.
The dated volcanic layers from the Salvan-Dorenaz basin set the
beginning of the detrital sedimentation at 308 +/- 3 Ma and constrain
the deposition of 1.5-1.7 km thick of elastic sediments within a time
span of 10-15 Ma. These results infer minimum, long-term subsidence
rates during basin evolution in the order of >0.1 mm/a, while in the
surrounding basement units estimated exhumation rates are in the range
of 1 mm/a. All dated rocks contain inherited zircon populations about
350, 450 or 600 Ma old.