Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 121

These rocks have a colour index of 60 first series is constituted of leucite

tephrite, tephritic leucitite, leucitite, phonolitic leucitite and leucite phonolite. This
series has been termed as a leucitite series by Ferguson and Cundari (1975). The
second series (phonolitic tephrite series) is constituted of leucite tephrite, phonolitic
leucite tephrite, tephritic leucite phonolite, latite and trachyte. Melabasanites are the
matrix of olivine, titanaugite and microcrystalline glass containing plagioclase and
minor amount of leucite. Leucite tephrites are the basic members grading from an
olivine-rich variety with subordinate amount of leucite to leucite-rich type with rare
amounts of olivine. Phenocrysts of titanaugite and ulvospinel occur with leucite
and/or olivine. Plagioclase is found to be restricted to the groundmass in equilib-
rium with leucite, titanaugite, Ti-magnetite. With disappearance of plagioclase, the
rocks become leucitites with the phenocrysts of leucite and titan-augite in a

75. The

-

ne-
grained matrix of leucite, titanaugite, Ti-magnetite, rare nepheline, olivine and
microcrystalline glass.
Leucite tephrites are the most basic rocks of the phonolitic tephrite series. The
tephrites are characterized by phenocrysts of titanaugite, olivine and rare ulvospinel
with microphenocrysts of titanaugite, plagioclase, leucite and Ti-magnetite ( glass).

In more evolved rocks, alkali feldspar is present and the leucite tephrite series grade
to phonolitic leucite tephrite and tephritic leucite phonolite. This series includes
xenoliths of clinopyroxene and olivine (

±

±

ulvospinel) or simply clinopyroxene,
olivine and biotite ( ± ulvospinel) and xenocrysts of biotite and rare kaersutite.
De Mulder (1985) made geochemical investigation of rocks of Birunga volcanic

field with particular emphasis on the lavas erupted by the extinct Karsimbi volcano,
the largest volcano in the Birunga field. According to De Mulder most of the K-rich

rocks of Karsimbi are either leucite basanite or K-trachyte. Different lava types of
this region are genetically related by fractional crystallization of observed pheno-
crystal phases from a parental basanite. The most primitive basanitic lavas with
high mg-number and Ni and Cr contents have the characteristics inherited from
mantle depths.
Bell and Powell (1969) considered that evolution of Birunga volcanics are
product of some crustal assimilation. Rogers (1992) however, thought that although
this may be true in case of K-rich mugearites, trachytes and some more Si-rich
variants (De Mulder et al. 1986), the negative high

field strength element anomalies

and lack of Eu anomalies in the ma c and near primary leucite basanites does not

suggest crustal assimilation in the evolution of magmas from this area.
Toro-Ankole is located about 125

155 km north-northeast of the Bufumbira

-

field (Fig. 4.9 , Brown 1971). The lavas of the area belong to the olivine leucitite and
olivine melilitite family and are completely devoid of feldspars. Katungites are also
fairly common. The lavas often include xenoliths of phlogopite peridotite, glim-
merite and partially fused granitic rocks. Chemical analyses of the leucite-bearing
rocks from the Western Rift are summarized in Table 4.7 .