Classification - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 76

calcite in minor amounts. In San Venanzo (Italy), a similar rock called venanzite,
consists of leucite (30 %), melilite (47 %), olivine (17 %) (all in vol%) and other
accessory phases (6 %). The name venanzite may be dropped as the term katungite
has been widely used. A rock consisting of kalsilite, augite, and olivine has been
called mafurite by Holmes (1950), the type locality of which is Birunga, Uganda.
A leucite-bearing ijolite has been called niligongoite by Holmes and Harwood
(1937). The rock is essentially composed of leucite, nepheline, clinopyroxene and
melilite with accessory olivine. Instead of the term niligongoite a more generalized
term such as melilite

nepheline leucitite may be used. If olivine is present as an
essential mineral, it should be called an olivine

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nepheline leucitite. Such
rocks are found in some localities of East Eifel, Germany and in the Nyiragongo
area of Birunga Province, Uganda.
A potassium-rich ankaratrite has been described by Sahama and Wilk (1952)
from the west of Nyamunuka crater in the Katwe-Kikorongo volcanic

melilite

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field of
southwestern Uganda. These rocks consist of clinopyroxene, olivine, nepheline,
leucite, biotite, apatite, perovskite, analcime, sodalite, and some opaque minerals as
accessories.

3.2.2 Leucitic Rocks with Feldspars

The term leucite tephrite refers to a rock comprising leucite, augite and plagioclase.
When the amount of leucite is signi cant it is called vesuvite (Holmes and Harwood

1937). Savelli (1967), Cundari and Mattias (1974) and Ferguson and Cundari
(1975) used the modal components, A (alkali feldspar)

F (felds-
pathoid) to classify leucite and feldspar-bearing rock types into leucite tephrite,
tephritic phonolite, phonolitic tephrite, phonolitic leucitite, tephritic leucitite, etc.
(Fig. 3.7 ). Various combinations of these rocks have been described by Ferguson
and Cundari from the Bufumbira region of Uganda. Such rock types have been
noted by Savelli (1967) from the Somma-Vesuvius region and by Cundari (1975)
from the Vico area (both in Italy). Bulk composition of these rocks would lie in the
simpli

P (plagioclase)

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SiO 2 (Sect. 9.2 ).
The essential minerals of leucite basanites include leucite, olivine, calcic pla-
gioclase, and clinopyroxene. The term kivite was used by Holmes and Harwood
(1937) to describe a leucite basanite with higher K 2 O/Na 2 O ratio. Bulk composition
of these rocks lies in the system forsterite

ed system diopside leucite anorthite

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anorthite. Apart from
the Bufumbira region, the intimate association of leucite tephrite and leucite ba-
sanite is found in the East Eifel region of Germany and different localities of Italy
(Chap. 4 ) .
Rocks consisting of augite, potash feldspar, biotite and rare plagioclase, have
been termed as shonkinite. Although it is free from leucite, shonkinite often con-
tains pseudoleucite, e.g. from Shonkin Sag, Montana (Weed and Pirsson 1896;
Nash and Wilkinson 1970, 1971) and Gentungen, Indonesia (Edwards et al. 1994).
Porphyritic rocks comprising olivine and augite in a groundmass of leucite,

- diopside - leucite -