Introduction - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 2

(a)

14

LH

12

HW

10

(b)

Lt Cald.

10

Rc

X

Wkim

9

8

8

EP

X

LA

7 Nav

6

NSW

Vul WEK

6

DVC

X

X X

5

X X

X

4

DSV

4

Eq.Af.

X

M

3

X

2

2

X

1

AB AB

0

40 50 60 10 20 30 40 50 60 70

SiO (wt %)
2

SiO (wt %)
2

: DSV = Deep Springs Valley (California, U.S.A.)
: M = Massif Central France

: WEK = Wudalienchih, Erkeshan and Keluo (N.E. China)
: DVC = Damodar Valley Coal Field (India)

: AB = Alkali basalt

: LA = Lower Austria
: EP = Eastern Paraguay

X

Fig. 1.1 a The K 2 O versus SiO 2 (wt%) content of potassic rocks from different localities. Wkim
West Kimberley, NSW New South Wales (both from Australia); LH Leucite Hills, HW Highwood
Mountains; Nav Navajo Hopi (U.S.A.); Vul Vulsini, Lt Cald Latera Caldera, Roccammfina (Italy);
Eq.Af Equatorial Africa; AB Alkali basalt. b The K 2 O versus SiO 2 (wt%) content of potassic rocks
from Deep Springs Valley (California, U.S.A.), Damodar valley (India), Wudalianchi, Erke-shan
and Keludo (Manchuria, China), lower Austria, Massif Central (France) and Eastern Paraguay

1.2 Minor, Rare Earth and Trace Element Characteristics

The minor, REE and trace element contents of potassium-rich silica-poor igneous
rocks from various localities were plotted by Foley et al. (1987) in chondrite-
normalised spider-grams (Fig. 1.3 a, b and c). The areas shown at the bottom of
these gures that the

incompatible elements in all the K-rich rocks from West Kimberley and Gaussberg
(Fig. 1.3 a), Toro-Ankole of S.E. Uganda (Fig. 1.3 b) and most localities of Italy
(Fig. 1.3 c) shows that barring Ti and Y, all other elements are markedly enriched in
the K-rich rocks compared to basalts of different localities.
O

figures depict MORB values. It may be observed from these

Brien et al. (1991) plotted 87 Sr/ 86 Sr versus ∈ Nd contents of potassic rocks

'

(Fig. 1.4 ), and found that the ∈ Nd concentration of MORB basalts are distinctly

higher (6.5 10), than that of potassic rocks (noted always below zero). The Sr

isotopic ratio of basalts (0.7020

-

0.7035) is much lower than that of silica-undersat-
urated potassic rocks from all the localities (e.g. Sunda Arc, Vesuvius, Roccamon

-

na,
Gaussberg, southern Spain, Leucite Hills and Smoky Butte; Fig. 1.5 ). In the case of
Spanish rocks, it is even greater than 0.716 and sometimes may be as high as 0.720.