85compositions (Brousse 1961a , b , 1971 ) and the geochemistry
of lavas (Maury 1976 ; Villemant 1979 , 1985 ).
The role of contamination (Glangeaud 1943 ; Javoy 1971 )
is deduced mainly from geochemistry and, in particular,
from isotopes (Briot 1988 , 1990 ).
The importance of magma mixing (Glangeaud 1943 ;
Gourgaud 1985 ; Gourgaud and Maury; 1984 ) is documented
by the presence, in some lavas, of two or more magmas
mechanically interstreaked together. Other evidences include
the presence, in a single lava, of phenocrysts crystallized
from two (or more) different magmas (e.g. basaltic and tra-
chytic) as well as the shape of geochemical trends (e.g. linear
correlation between elements with distinct behaviours).
The Monts Dore massif is dominated by lavas of interme-
diate composition, the so-called trachyandesites of the over-
saturated and undersaturated series. These are explained by
the mixing of mafi c magmas such as basalts from deep levels
with more differentiated trachytic or phonolitic melts from
shallow levels in the volcanic plumbing system. Two discrete
magma reservoirs are believed to have existed beneath the
Monts Dore: a deep reservoir where basaltic magma was
stored, and a shallow-level one where more differentiated
magmas, such as rhyolitic, trachytic or phonolitic ones
evolved by fractional crystallization coupled with crustal
contamination. In some cases, mafi c magmas ascending
from depth were injected into the base of the shallow reser-
voir, causing mixing of magmas of disparate compositions
and temperatures. If mixing was incomplete, the resulting
lava is highly heterogeneous, with occurrence of cognate
enclaves, banded and emulsifi ed textures (Gourgaud 1985 )
such as observed at the Grande Cascade (Gourgaud and
Villemant 1992 ). More complete mixing generated hybrid
lavas that are macroscopically homogeneous. A majority of
Monts Dore trachyandesites are believed to originate from
such mixing processes.
4.3 Eruptive Styles
4.3.1 Eruption of Effusive (Flows), Extrusive
(Domes) and Intrusive (Dykes) Lavas
The products of these styles may or may not exhibit colum-
nar jointing. Sub vertical columns are characteristics of the
fl at-lying fl ows such as the Clé du Lac basalt, near lac de
Guéry. Horizontal columns are characteristics of dyke mar-
gins, such as the trachyte of the Monnéron quarry near la
Bourboule. The inner part of eroded phonolitic domes also
exhibit large vertical columns, for example Roche Tuilière.
The margins of phonolitic domes exhibit columns arranged
in a fan shape, as seen at the base of Monnéron quarry.
4.3.2 Explosive EruptionsThe products of explosive eruptions include pyroclastic
fl ows, fall and surges. Debris avalanches could also be
related to explosive activity, even though the juvenile magma
is not identifi ed in their deposits.
The main types of pyroclastic fl ows found in the Monts
Dore include ignimbrites (ash-and-pumice fl ows) and
nuées ardentes (block-and-ash fl ows). The deposit from
the largest eruption is the “Grande nappe” ignimbrite, with
a wide extension and an estimated volume of about
8–10 km^3 (Brousse 1961a , b ; Mossand 1983 ). This ignim-
brite is composed of ash, fi brous rhyolitic pumice and
xenolithic fragments issued from the basement. The
smaller pumice fl ows of Rioubes Haut and Neschers are
trachytic in composition and were erupted from the Sancy
volcano. The nuée ardente deposits of Puy de la Tache,
Gacherie and Durbise were formed by lateral explosions
from lava domes.
The ash fall deposits of the so-called Guéry series (Morel
1987 ; Cantagrel and Briot 1990 ) are locally reworked by
water (cinerites). They contain layers of well stratifi ed ash
and are associated to pyroclastic fl ows. These eruptions were
emplaced between 2.19 and 2.08 Ma (Nomade et al. 2014b )
and found as far as the Senèze maar sequence 60 km SE of
the Monts Dore massif (Nomade et al. 2014a ). Pyroclastic
surges of the Guéry series were formed by hydromagmatic
eruptions (Morel et al. 1992 ), probably in a subglacial
environment.
Debris avalanche deposits were formed by the instability
and sliding of the Mont Dore s.s stratovolcano (Aiguiller
area). Such events have been recently dated at 2.58 Ma
(Nomade et al. 2014a , b ). These avalanches reached to the
east as far as the Allier valley at Perrier, near Issoire. The last
one continued to the north as far as 50 km. Probably, these
avalanches transformed progressively downstream into mud-
fl ows (lahars). Traces of them are found among the ancient
alluvial terraces of the Allier river near Vichy, as far as 60 km
from Perrier. In the Sancy volcano, debris avalanche deposits
occurred too, and were related to the collapse of the eastern
fl ank of the volcano, more than 900 ka ago.4.4 Geological History of the Monts Dore
MassifThe complex history of the Monts Dore massif is related to a
long-lasting activity over about 3.1 Ma, to the variety of
erupted lavas (two complete alkaline series) and the diversity
of eruptive styles. The main events are illustrated in Figs. 4.1
and 4.2.4 Volcanism of the Monts Dore (French Massif Central)