Genesis of Ultrapotassic Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 446

(1) a basaltic magma and granitic rocks (Turner and Verhoogan 1960),
(2) a basaltic magma and peridotitic rocks (Holmes and Harwood 1932, 1937),
(3) a carbonatite magma and granitic rocks (assimilation)(Holmes 1965),
(4) a monchiquite magma with granitic rocks (Williams 1936),
(5) a trachytic magma and carbonate-bearing sediments (Rittmann 1933).

s argument (1928) seems to be
very valid. He pointed out that at the time, any magma punctures through the
earth

In case of all assimilation hypotheses, Bowen '

is crust it is not superheated enough to assimilate foreign materials and avoid
concomitant crystallization. It seems highly unlikely that after assimilation, they
should be completely modi

'

ed to a new magma type and then should be still hot
enough to undergo fractionation so that different daughter products of a K-rich
magma could be generated (see also Wyllie 1974).
Regarding the first assimilation hypothesis, it should be further mentioned that

this hypothesis does not explain the high Ni, Co, Cr, Sc content of many K-rich
ultrama

c rocks, as neither granites nor basalts are enriched in these elements.
Experimental study with four compositions involving a mixture of dolerite and
granite in following variable proportions were made: 80:20, 60:40. 40:60 and
20:80. Experiments were conducted in one atmosphere between 800 and 1,000

°

C

°

(Yagi and Gupta 1977) and at 750 C under 0.1 GPa. The run products failed to

produce any feldspathoid-bearing assemblage.
Holmes (1950, 1965) argued that assimilation reaction between a carbonatitic
magma and granitic rocks could produce highly potassic silica-undersaturated
magma like katungite (Bell and Powell 1969). Carbonatites are enriched in
incompatible elements and LREE, and thus such an assimilation hypothesis could
explain observed trace and minor element distribution pattern. The P-T

T stability of
carbonates and other minerals in carbonatites, however suggest their low temper-
ature of formation (Koster van Groos and Wyllie 1973). The eruption temperature
of carbonatitic liquids at Oldoinyolengai, Tanzania was only around

-

C
(Dawson 1966). Thus, a magma with such a low temperature of eruption, before
incorporating any other rock type should undergo concomitant crystallization
before being transformed into a potassic magma type. In many regions such as
Mundwara (Rajasthan, India), Ambadongar, Gujarat (India), West Eifel (Duda and
Schminke (1985) and Kaiserstuhl (Wimmenauer 1974), both the places are from
Germany), carbonatites invading granitic country rocks are found to be in close
association with fenites and Na-rich undersaturated rocks; and in these localities
potassic rocks are not found. Besides, in most localities ultrapotassic rocks are
found to occur independent of carbonatites.
In cases of assimilation hypotheses involving carbonate sediments and trachyte
or monchiquite and granitic rocks, it may be noted that trachytes and monchiquites
themselves are of secondary in nature, produced by fractionation of other primary
magmas and therefore, are not hot enough to assimilate other rock types, and then
undergo further fractionation to produce ultrapotassic rocks. Such a process should
not also explain enrichment of mantle compatible elements like Ni, Co. Cr, etc.
found in many K-rich ultrama

500 °

*

c rocks.