Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 203

(a)

Azores

Madeira

30 0

Canary
1st.

AFRICA

Cape
Verde
1st.

0 0 0 0

Guinea
1st.

Fernando
Noronha

SOUTH
AMERICA

St. Helena

Trindade

30 0

Tristan Gp.
Gough

Bouvet

60 0 30 0 0 0

(b)

Fig. 4.34 a Location of Tristan da Cunha Island (after Borley 1974). b Map of Tristan da Cunha
modi ed after baker et al. (1964), the shaded refers to Scoria deposits. c Plot of chemical
composition of Tristan Da Cunha rocks in a total alkali versus SiO 2 diagram (after Brenna et al. 2014)