Genesis of Ultrapotassic Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 448

lavas and therefore himself, could not be genetically related. Holmes therefore,
disproved his own hypothesis.
Further, with reference to the second hypothesis, even if suf

cient heat is pro-
vided to a region having a gabbro and peridotite interface by a mantle plume, the
liquid thus, produced should not be enriched in Ba, Rb, Sr, Zr etc., as either of the
two rock types are impoverished in these elements.

15.3 Zone Re ning Hypothesis

Zone re ning hypothesis of Harris (1957) suggests that incompatible elements have

very low K D values [X tr (liquid/crystal

0.02)], hence during partial melting,
such elements as K, Rb, Sr, Ba, etc. should be saturated in the

≤ 0.01

-

first liquid after low
degree of melting, but this mechanism would have produced K-rich rocks as an
exception rather than a rule.

15.4 Genesis of Potassic Rocks by Volatile Transport

Lindgren (1933) considered that differentiation largely through gaseous transport
produced trachytes, phonolites, basanites and many other volcanic rocks. Rittmann
(1933) also suggested this mechanism together with assimilation between limestone
and trachyte to be an effective way of producing leucite-bearing rocks of Vesuvius.
Morey and Hesselgesser (1952) showed that 40 % by weigh of alkali silicates could
be dissolved by aqueous vapour phase at 400 C and 2,200 bar. From these

observations, Kennedy (1955) considered that high solubility of alkali silicates in
gaseous water indicates that they may strongly associate with water molecules and
their solubility is very much dependent on pressure and temperature. Thus, when
there is a release of pressure, resulting from fracturing during escape of volatiles,
alkali silicates should be left behind because of decrease of solubility. Concentra-
tion of alkalis at the top of an igneous stock may result from such a process, as in
the case of potash enrichments in igneous rocks of Jumbo basin, Alaska (Kennedy
1955) or incase of formation of pseudoleucite crystals in the contact zone of
shoshonitic dyke and country rocks in the Highwood Mountains region (Larsen
et al. 1941).
In case of this gaseous transport hypothesis alkaline rocks should have been
emplaced always at the roof tops in isolated cupolas during the last stage of vol-
canism. Such a process could lead to accumulation of alkaline rocks locally. This
process cannot explain large scale lava

°

flows found in the surface in many localities

throughout the world.