Classification - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 72

Fig. 3.4 K 2 O/Al 2 O 3 versus
mg-number variation in
ultrapotassic rocks (after
Foley et al. 1987)

1.6

1.4

1.2

Group I

1.0

0.8

0.6

Group II

0.4

Group III

0.2

0.0

40 50 60

100Mg/ (Mg+Fe)

70 80

Fig. 3.5 CaO versus MgO
variation in ultrapotassic
rocks (after Foley et al. 1987)

18

16

14

Group III

Group II

12

10

8

Group I

6

4

2

0

0 2 468101214 16 18 20 22 24 26 28 30

MgO (Wt. %)

common. High SiO 2 content does not allow crystallization of such silica-undersat-
urated accessory minerals, as kalsilite, melilite, perovskite, priderite and wadeite. The
nodules are clinopyroxene-rich rocks, and have been described from Alban Hills,
Roccamon na and Somma-Vesuvius.

The group IV rocks are those, which cannot be included in either of the three
groups. The type examples of group IV rocks are the potassic rocks of Navajo
volcanic field, which lack mantle-derived xenoliths.

The major problem related to this type of grouping of K-rich rocks such as group 1,
group 2 (K-rich rocks), group 3 (K-rich rocks) and group 4 (K-rich rocks) (I, II, III
and IV), lies in the fact that the rocks of all four groups have been described from