Geology Reference
In-Depth Information
(Downes 1984, 1987a; Briot 1988; and Briot et al. 1991)]. Rare nephelinites also
occur but have markedly different geochemical characteristics from the basanites
and alkali basalts and constitute a separate group, not associated with differentiated
lavas. The nephelinites vary widely in their K
2
O/Na
2
O ratio ranging from sodic
melilite nephelinites (K
2
O/Na
2
O < 1) to potassic leucite mela-nephelinites (K
2
O/
Na
2
O > 1). Representative analyses of the main ma
c lava types are presented in
Table
4.16
.
Leucite mela-nephelinites have K
2
O contents higher than those of normal ba-
sanites and alkali basanites on the total alkali-silica (TAS) diagram of Le Bas et al.
(1986). In contrast, the melilite nephelinites have low total alkali contents. Basa-
nites plot on the TAS diagram between 40 and 46 wt% SiO
2
, and total alkali
content varies between 5.0 and 6.6 wt%, whereas alkali basalts plot between 42 and
48 wt% SiO
2
and 4 and 6 wt% total alkalis.
Concentration of TiO
2
,P
2
O
5
,K
2
O and Na
2
TiO2, is generally higher in the basanites
than in the alkali basalts, although Al
2
O
3
and MgO are similar. The nephelinites,
however, have signi
cantly lower Al
2
O
3
contents (9
-
10 wt%) compared with
basanites and alkali basalts (12
-
15 wt%). The K
2
O/Na
2
O ratios fall in the range of
0.3
0.7 for typical basanites, alkali basalts, and melilite nephelinites, whereas the
leucite nephelinites fall in the range of 1.2 and 1.7. The Massif Central is a region of
uplifted and eroded Hercynian metamorphic basement, composed mainly of
granitoids, schists, and amphibolites. The major phase of rifting (Limagne graben)
occurred during the Oligocene, and was associated with regional uplift. Minor
volcanic activity between 0.5 and 35 Ma pre-dated graben formation and involved
the eruption of small volumes of under-saturated alkalic lavas (mostly nephelinites)
along basement faults (Lucazeau et al. 1984). Volcanic activity decreased consid-
erably during the main phase of graben formation and then resumed during 20 to
*
-
0.04 Ma ago (early Miocene to late Quaternary) with major phases at 7
4Ma
-
(Pliocene) and 4
2 Ma (Viilafranchian). Within the Massif Central there are many
individual volcanic provinces, which include, Cantal
-
(11
3 Ma), Mt. Dore
-
(4.3
0.4 Ma), Aubrac (8
6 Ma), Deves (5.4
0.8 Ma), Velay (10
1 Ma) and Chaine
-
-
-
-
des Puys (50
c magmas are predominantly alkali basalts and
basanites although nephelinites are present in a few localities (Downes 1987). The
nephelinites include both sodic (melilite nephelinites) and potassic (leucite mela-
nephelinites) types. There may be systematic difference in the Na
2
O/K
2
O ratio of
primitive magmas erupted in different volcanic areas but at present
4 Ka). Primitive ma
-
there are
insuf
cient data to substantiate this hypothesis.
4.8.3 K-Rich Rocks from Lower Austria
In the southern Bohemian massif (central Europe) veins of medium to coarse-
grained glimmerites occur in peridotite massifs and at the contact with large bodies
of felsic granulites (Becker et al. 1999). Crystallization and metamorphism of the
glimmerites and the granulites were coeval. The glimmerites contain phlogopite and
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