Lake Pavin History, geology, biogeochemistry, and sedimentology of a deep meromictic maar lake

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the thickness of the tephra bed that attains 10–30 cm at the
easternmost sites, and 3 cm at the southernmost site (Planèze
de Saint Flour). Due to the lack of peat bogs outside the
investigated zone, and thus no opportunity to assess its
occurrence and thickness beyond this zone, it is nevertheless
reasonable to assume that the lobe must extend much further
eastwards and southwards. The minerals consist essentially,
in order of decreasing frequency, feldspars, brown amphi-
boles, green clinopyroxenes, and opaque Fe-Ti oxides.
Various xenoliths from the basement and volcanic products
of the Monts Dore are also present (lithics).
Microprobe analyses were performed to constrain the
geochemical composition of glass shards in a sample of the
tephra from the La Godivelle area. That enabled its determi-
nation in the TAS classifi cation (glass basis) as a trachyte
(Fig. 8.4 ). Total rock analyses of proximal products were
performed by Bourdier ( 1980 ); since minerals were part of
the sample, the magma is less differentiated than the glass
and it was determined to be (whole-sample basis) trachyan-
desite and basaltic trachyandesite.

B. Northern lobe

So far the tephra layer is known only at three localities
(Fig. 8.3b ): the Narse d’ Ampoix (Juvigné and Gewelt 1987 ),

the Narse d’ Espinasse (Juvigné et al. 1988b ), and the lake of

the Gour de Tazenat (Juvigné and Stach 1998 ). Juvenile

magma is represented by light grey pumice clasts up to

1.6 mm in diameter at Ampoix and Espinasse, and by bubble-

wall shaped glass shards smaller than 0.3 mm at Tazenat.

That decreasing size trend (from Ampoix and Espinasse to

Tazenat) is matched by a trend of decreasing thickness, the

layer being 1 mm thick at Ampoix and 0.5 mm at Tazenat. In

the mafi c mineral suite, brown amphiboles exceed green

clinopyroxenes by a ratio of approximately 4:1. A few

titanites, apatites, and brown micas are also present. Beyond

the investigated area it can be assumed that the lobe should

overlap more regions farther eastwards and/or westwards

while farther northwards the tephra is close to the limit of

detection.

Microprobe analyses of glass shards were performed

on samples of tephra from the Narses d’ Ampoix and

d’ Espinasse bogs as well as from the lake sediments at

Gour de Tazenat. Those analyses enabled the glass shards

to be determined in the TAS classifi cation as trachyte;

nevertheless two populations can be clearly distinguished

based on silica and alkali content (Fig. 8.4 ). The joint

presence of those populations can be explained either by a

single pulse involving magma mixing, as shown by banded

pumices in proximal sites of the volcano (Leyrit, personal

Fig. 8.4 Determination of the magma composition of the Montcineyre ,
Montchal and Pavin* volcanoes after the TAS classifi cation (Le Bas
et al. 1986 ). () Total rock analyses of ash after Juvigné and Bastin
( 1995a ), and lava fl ows after Bourdier ( 1980 ); (**) glass analyses of ash
after Juvigné and Stach ( 1998 ) and Gourgaud for glass shards of the
southern lobe (unpublished), and total rock of one pumice and one cau-
lifl ower bomb after Bourdier ( 1980 )

Legend of TAS-classifi cation (Le Bas et al. 1986 ). F foïdite, Pc picro-

basalte, U1 basanite, U2 phonotephrite, U3 tephriphonolite, Ph phono-

lite, B basalt, S1 trachybasalte, S2 basaltic trachyandesite, S3

trachyandesite, T trachyte, O1 basaltic andesite, O 2 andésite, O3 dacite,

R rhyolite

8 Distribution, Tephro- and Chronostratigraphy of the Widespread Products of Pavin Volcano