Geology Reference
In-Depth Information
3.1.4 Major Oxides as Basis of Classi
cation
Foley et al. (1987) classi
ed potassic rocks into four groups: group I, group II,
group III and group IV. The group I lamproites (Leucite Hills type of Barton (1979)
and Sahama
'
s orendite class) are characterized by low CaO, Al
2
O
3
and Na
2
O
contents and high K
2
O/Al
2
O ratio usually above 0.6 but frequently >1, and mg-
number as high as 87 (Figs.
3.3
,
3.4
and
3.5
). These rocks have high incompatible
trace element contents and include peridotitic mantle xenoliths. The lherzolitic
types are less predominant than the garnet and clinopyroxene-poor varieties, which
have lost the basaltic melt component. Two type localities of group I rocks include
West Kimberley and Gaussberg. According to Foley et al. group I rocks usually
have no associated non-potassic rocks, but non standard members have calc-alka-
line and shoshonitic associates. Group I rocks may occur in orogenic areas, but are
usually reported from the stable continental areas.
The kamafugitic rocks, included as group II rocks, have low SiO
2
(<46 wt%) and
high CaO. They have a low Al
2
O
3
, less extreme K
2
O and have lower K
2
O/Al
2
O
3
ratio. Mg-number is usually above 60. Incompatible elements are less-enriched than
the rocks of group I, but have a positive Sr spike. The typical localities include Toro
Ankole (southeastern part of Uganda), Cupaello and San Vananzo (Italy). They are
supposed to be associated with rift valleys. These rocks are characterized by the
presence of melilite, perovskite and kalsilite. The mantle nodules included within
group II lavas, are rocks of olivine
pyroxene (O.B.P.) series.
The Roman Province type rocks, included within group III, are characterized by
high Al
2
O
3
content (Fig.
3.3
), lowK
2
O/Al
2
O
3
ratio (generally <0.5, Fig.
3.4
), and the
K
2
O concentration is higher than the group II rocks. Mg-number is lower than the
other two groups of rocks (Fig.
3.4
, rarely above 70), and rocks with extremely low
silica contents (<42 wt%) do not occur, but those having less than 50 wt% are
common. The type locality is Roman Province of Italy and Indonesian province.
Because of high Al
2
O
3
content, plagioclase and alumina-rich clinopyroxenes are
biotite
-
-
Fig. 3.3 Na
2
O versus Al
2
O
3
variation in ultrapotassic
rocks (after Foley et al. 1987)
22
20
Group III
18
16
14
12
10
Group I
Group II
8
6
4
2
0
0
1
2
3
4
5
Na O (Wt. %)
2
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