Classification - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 73

close proximity, and thus, there is no genetic signi cance. For example, petro-

graphically the rocks of Bufumbira can be classi ed into three series: (1) leucitite

series (olivine + leucite + clinopyroxene and glass), (2) leucite tephrite and leucite
basanite series (containing variable proportion of leucite, olivine, augite, ca1cic
plagioclase and glass), and (3) the potash trachybasalt (absarokite, shoshonite)
series comprising olivine, augite, K-feldspar and glass.
Whereas the rocks of leucitite series can be considered as group I rocks
according to the suggestion of Foley et al. (1986), those belonging to second and
third series (tephrite

— basanite series and trachybasalt series) should be similar to

their group III rocks.
The volcanic rocks of Toro-Ankole, which are located only 100 km away, have
been designated as group II rocks by them. Thus, group I, group II and group III
rocks, are no doubt the transitional rock types (group IV), all located in close
proximity.
Conticelli and Peccerillo (1992) described the presence of lamproites (orendites,
group I), kamafugitic rocks (group II), high alumina and relatively silica-rich rocks
of K-series (tephrites and phonolites, group IV) and transitional types (group-IV
rocks) in central Italy.
According to Foley et al. (1987) the group I rocks might have been formed from
a depleted mantle source with some rocks enriched in F, H 2 O, and CH 4 , but
impoverished in CO 2 . The group II rocks are considered to have been formed by
partial melting of a mantle source, rich in CO 2 but poor in H 2 O. According to them
group IV rocks might have been formed under orogenic conditions. The coexis-
tence of all the four groups of rocks in central Italy, suggest that all the diverse
physico-chemical conditions were surprisingly present within such a limited area.
Thus, classi

cation of K-rich rocks into group I, group II, group III and group IV,
does not appear to be useful.

3.2 Classification on the Basis of Mineralogy

There is an incompatible relationship between melilite and plagioclase in the lavas
of alkalic suites (Yoder and Schairer 1969; Yoder 1973). From volcanic centres of
the same petrographic province, either melilite-bearing lava is erupted, or plagio-
clase-bearing tephritic rocks are ejected. Petrologic study of the melilite-bearing
potassic rocks and leucite-bearing basanites from the Eifel support the conclusions
of Yoder and Schairer (1969). From his study of natural rocks from various areas,
Yoder (1973) established that in alkaline lavas, melilite and plagioclase do not
coexist but these two minerals may coexist in metamorphic rocks.
Potassic rocks therefore, have been classi

ed into following categories in this

volume:
(1) Those, which are free from plagioclase and crystallized under low P(H 2 O)
condition (melilite may or may not be present),