87Despite the good geologic knowledge acquired since the
1960s (Brousse 1961a , b , 1963 , 1974 ; Vincent 1985 ; Pastre
and Cantagrel 2001 ) and developed in numerous theses from
Paris-Orsay and Clermont-Ferrand universities, a complete
consensus has not emerged yet (Nomade et al. 2012 , 2014a ,
b ). The debate still focuses on the tephrochronology of the
numerous pyroclastic deposits (mainly pumice fall and
fl ows), on the location and shape of the Haute-Dordogne cal-
dera , the signifi cance of the pumiceous linked to its collapse,
and on the debris-avalanche deposits produced by two gen-
erations of fl ank failures in the Monts Dore (Aiguiller area)
and Sancy stratovolcanoes.
The pre-volcanic basement of Monts Dore is essentially
metamorphic and granitic in nature and Paleozoic (Variscian)
in age, but it includes several small Permian, Oligocene and
Miocene sedimentary basins. Before the Monts Dore growth,
the area was already uplifted as shown by basement outcrops
between 900 and 1200 m asl, below the volcanic rocks.
Volcanic activity prior to the Monts Dore massif (20–3.2
Ma) emplaced Miocene alkali basaltic and basanitic lava
fl ows similar to those of the Limagne basin as well as a few
trachytes, phonolites, and rhyolites, forming a widely scat-
tered alkaline series of Pliocene age (Fig. 4.2a ).
4.4.1 The Monts Dore Stratovolcano
(3.07−1.46 Ma)
The history of the Monts Dore s.s. stratovolcano began with
the collapse of the Haute Dordogne caldera, a depression
5 km wide and 250 m deep. Now, it is no longer visible due
to infi lling by later activity. Distinct caldera boundaries have
been delineated (Brousse 1961b , Mossand et al. 1982 ;
Vincent 1981 , 1985 ; Cantagrel and Briot 1990 ) based on
geologic evidence and tectonic and geophysical data (Bayer
and Cuer 1981 ; Nercessian et al. 1984 ). However, a common
boundary has been drawn by all authors around two locali-
ties: La Bourboule, to the NW, at the contact between granite
and pyroclastic formations (considered as hydrothermalized
pumiceous infi lling of the caldera), and at the foot of the
Charlannes plateau, to the SW, at the contact between the
pre-caldera lavas and similar pyroclastic formations.
The Haute Dordogne caldera collapse is attributed to the
discharge of voluminous rhyolitic pyroclastic fl ows that
formed the 8–10 km^3 (bulk volume) “Grande Nappe” ignim-
brite (Boudon and Mossand 1984 ). This eruption occurred
about 3.07 Ma ago. The ignimbrite, with typical rhyolitic
fi brous pumices and quartz phenocrysts, has been found
30 km away from the centre of the caldera , and crops out in
the quarry at Rochefort-Montagne, where its thickness
exceeds 10 m, more than 10 km away from its postulated
source.
The eruption of quartz-rich porphyritic pumices (essen-
tially plinian falls), rich in quartz and termed “Roca Neyra”
(3.09 Ma, Nomade et al. 2014a , b ), pre-dated that of the
Grande Nappe ignimbrite, although age on both formations
overlap. Quartz phenocrysts are similar to those of the
Grande Nappe, but the “Roca Neyra” extension is limited
towards the eastern part of Monts Dore area. Both deposits
of Grande Nappe and Roca Neyra may belong to the same
eruptive cycle, fi rstly plinian, then ignimbritic, a feature
which suggests an incremental caldera collapse or possibly
two successive caldera-forming eruptions.
Following the establishment of a lake, the Haute Dordogne
caldera depression was fi lled with lacustrine sediments. Then
followed a period of intra-caldera volcanism involving the
eruption of rhyolitic domes (and associated block-and-ash
fl ows) of trachy-phonolitic domes, trachandesitic lava fl ows,
dated between 2.86 ± 0.02 Ma and 2.60 ± 0.02 Ma (Nomade
et al. 2014a , b ). This period of intense activity ended by four
large debris-avalanche (and associated lahars) that formed
the actual Perrier Plateau near Issoire (Besson 1978 ; Ly
1982 ; Pastre 1987 , 2004 ; Pastre and Cantagrel 2001 ). These
events were followed by the emplacement (dated between
2.40 and 2.10 Ma) of large rhyolitic and phonolitic intrusions
in the central part of the volcano (e.g. Puy de Chantauzet,
Roche Sanadoire; Mossand et al. 1982 ; Cantagrel and
Baubron 1983 ).
Major phreatomagmatic eruptions from the Aiguiller
vents (Morel et al. 1992 ) generated the pyroclastic-fl ow and
plinian-fall deposits of the “Guery series” (Cantagrel and
Briot 1990 ), dated at 2.19–2.08 Ma (Nomade et al. 2014a , b ).
The activity of this fi rst strato-volcano, the Monts Dore
s.s. , ended around 1.46 Ma ago (Fig. 4.2c ).
Recent tephrochronological data (Nomade et al. 2014a , b )
emphasized four cycles of explosive activity (mainly pum-
ices), according to proximal and distal records, between the
caldera collapse and rhyolitic ignimbrite (G.I.: 3.09–3.02
Ma) and the end of activity (1.46 Ma). G.II occurred between
2.86 and 2.58 Ma, and G.III, around the Guery area, between
2.36 and 1.91 Ma. G.IV corresponds to a small-volume unit
dated at 1.46 Ma.4.4.2 The Sancy Stratovolcano (1.1–0.23 Ma)After about 400,000 years of dormancy, a new stratovolcano
grew, 10 km south of the previous volcanic centre, on the
southeastern fl ank of the Monts Dore s.s. stratovolcano. The
Sancy covers about 16 km^2 with a present-day summit at
1886 m (Puy de Sancy). The Sancy was the most active vol-
cano of the French Massif Central during the late Early to
Middle Pleistocene periods (Pastre and Cantagrel 2001 ). Its
activity ended around 230 ka ago according to K/Ar ages4 Volcanism of the Monts Dore (French Massif Central)