1137 % clinopyroxenes (including augite and aegyrinic augite),
6 % plagioclases (andesine) and 3 % opaques. The ground-
mass includes 52 % glass and 22 % microcrysts with 15 %
feldspars, 3 % hornblende, 3 % clinopyroxene, 2 % apatite,
1 % sphene and 1 % opaque (Melet 2009 ; Lêvèque and
Vaillant 2010 ; Jaillard and Zylberman 2012 ). Microscopic
textural criteria indicate two plagioclase populations: the
first typically show disequilibrium textures such as sieved,
spongy and different types of oscillatory zoning (2a and 2b,
Fig. 6.5). Abundant melt inclusions varying from elongate to
circular shapes result in coarse sieved texture. The second
population represents an equilibrium texture with normal
oscillatory zoning. Pumices can contain two main forms of
vesicles: tubular microvesicles that can impart a fibrous fab-
ric, and spherical to subspherical vesicles resulting from
high vapor pressure during eruption. Sometimes glass splin-
ters can be observed within the matrix, in particular on the
periphery of vesicles. It is also important to notice that the
vesicle content can vary widely. This was already noted by
Bourdier ( 1980 ) who identified dense juvenile pyroclasts in
some parts of the deposit.6.4.1.2 Lithic Pyroclasts
The lithic pyroclasts consist of particles from the crystalline
basement and old lavas. Three types of basement clasts
(noted B) are identified: (1) cordierite-bearing migmatite (2)
muscovite leucogranite with an assemblage of musco-
vite + biotite + perthitic microcline + oligoclase + quartz and
(3) sillimanite-biotite-garnet gneiss with foliated texture
made up of granular mineral grains, with an assemblage
composed of rare cordierite + late muscovite + oxides + gar-
net + sillimanite + biotite + perthitic microcline + oligoclase +
quartz (Bourdier 1980 ; Lêvèque and Vaillant 2010 ). The old
lavas are subdivided in dense lavas (noted DLP for Dense
Lithic Pyroclasts) and black to red scoria (noted VLP for
Vesiculated Lithic Pyroclasts).Fig. 6.5 Photomicro graphs
of the Pavin’s juvenile melt.
(a) is the PPL
photomicrograph and (b) is
the same photograph in XPL.
(1) Typical paragenesis
showing brown hornblende
(Hb), clinopyroxenes (Cpx),
plagioclase (Pl), apatite
inclusions (Ap) and opaques
(O). (2) Destabilization
structure in a twinned and
zoned plagioclase. (3)
Shearing of an amphibole
during the phreatomagmatic
eruption
6 Characterization of Phreatomagmatic Deposits from the Eruption of the Pavin Maar (France)