Classification - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 68

Chapter 3
Classification

Potassium-rich silica-undersaturated igneous rocks may be classi ed primarily on

the basis of their chemistry and mineralogy, but on petrological stand point the
mineralogical classification is more preferred here particularly in view of plethora
of rock names given on the basis of localities from which these rocks have been
described. Their nomenclature is therefore, not only confusing but also complex at
times. Different types of classi cation schemes are described below.

3.1 Classification Based on Chemistry

3.1.1 Potassium Content as a Basis of Classi cation

The K-rich rock types have been described into three series by Appleton (1972): (1)
low-K, (2) medium-K and (3) high-K series on the basis of a simple plot of K 2 O
versus SiO 2 (Fig. 3.1 ). The distinction of three different series has been made on the
basis of three statistically discriminating lines de ning different degrees of potas-

sium-enrichment.
This scheme has been extensively used to classify rocks from Italy by Rogers
et al. (1985), from Highwood Mountains (Montana) by O ' Brien et al. (1991) and

from Ringgit - Beser complex, Indonesia by Edwards et al. (1994).

3.1.2 Total Alkali Versus Silica Classi cation

Both sodium- and potassium-rich rocks have been classi ed by the IUGS sub-

commission on the systematics of igneous rocks into phonolite, tephri-phonolite,
phonotephrite, tephri-basanite and foidite. This has been done on the basis of a
(Na 2 O+K 2 O) versus SiO 2 plot (Fig. 3.2 ). It is also known as the total alkali versus
SiO 2 (TAS) diagram. Le Bas et al. (1986) used straight lines to de

ne different