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4.1 Introduction
The Monts Dore massif (maximum elevation 1886 m a.s.l. at
Puy de Sancy) covers an area about 500 km^2. Its volcanic
activity produced about 200 km^3 of eruptive material. This
massif consists of two distinct edifi ces, the Monts Dore s.s.
and the Sancy, according to Baubron and Cantagrel 1980 ;
Cantagrel and Baubron 1983 (Fig. 4.1 ).
The Monts Dore s.s. stratovolcano is 3.09–1.46 Ma old.
Its volcanic activity began by an ignimbritic eruption of rhy-
olitic ash-and-pumice fl ows, 3 Ma ago. As a result of this
paroxysmic eruption, a caldera collapse event occurred,
forming the “Haute Dordogne” caldera. In the vicinity of this
caldera, lava fl ows, dykes, domes (and associated block-and-
ash fl ow deposits) and minor pumice fl ows were emplaced.
They are rhyolitic, trachytic, trachyandesitic, basaltic, teph-
ritic and phonolitic in composition.
The Sancy stratovolcano is younger, 1.1–0.23 Ma old. It
is centred on a smaller caldera related to the outpouring of
trachytic ash-and-pumice fl ows, about 720 ka ago.
Trachyandesites are predominant (75 % in volume) and were
emplaced as domes, pyroclastites, lava fl ows and dykes.
Basaltic lava fl ows mainly erupted in the distal part of the
stratovolcano.
The petrology of the two distinct volcanoes is almost
identical and involves the coexistence of two types of alka-
line series, which evolved either to silica-oversaturated (rhy-
olitic) or to silica-undersaturated (phonolitic) residual
liquids. The main magmatic processes controlling their evo-
lution are fractional crystallization, magma mixing and
crustal contamination.
Pavin volcano is located in the Sancy area. So the prod-
ucts of the volcanic activity of Pavin may include xenolithic
material mainly originated from the Sancy volcano: trachy-
andesite, basalt, trachyte and scarce phonolite.4.2 Monts Dore Magmas
Magmas of the Monts Dore are of alkaline intraplate type
and are potassic in nature. There are two co-existing magmas
series: the silica-oversaturated series (from basalt to rhyolite)
and the silica-undersaturated series (basanite to phonolite).
Three kinds of processes controlled the genesis of the Monts
Dore lavas: fractional crystallisation, crustal contamination
and magma mixing.
Evidence for fractional crystallisation is provided by the
presence of cumulate enclaves in some lavas, the mineralFig. 4.1 Geological map of the Monts Dore massif (After Cantagrel and Baubron 1983 , modifi ed)
A. Gourgaud