Geology Reference
In-Depth Information
K-rich volcanic rocks. It has been described form mafuritic rocks of Uganda and
also from the Alto Para-naiba igneous province of Brazil. Kalsilite-bearing lava has
been described from Mt. Cimini also.
A nepheline-kalsilite-bearing intrusion occurs at Pkhrutskii (Caucasus). This
intrusive covers an approximate area of 10 km
2
and can be classi
ed as a nepheline-
kalsilite-bearing monzonite, which locally grades to essexite and
fine-grained
syenite. The main rock-forming minerals are nepheline, kalsilite, K-feldspar, augite,
aegirine-augite and biotite.
A pseudoleucite-bearing alkali syenite occurs at Yaksha, Baikal (Russia). The
syenite comprises pseudoleucite (up to 20 vol%) and nepheline. Nepheline-kalsilite
intergrowth is often seen in thin section. Locally the syenite grades to kalsilite
syenite. This intrusive body constitutes 65
75 % K-feldspar, 3
10 vol% biotite and
-
-
up to 2 vol% kalsilite.
Predominantly katungites and mafurites have been described from Quaternary
volcanic complex at San Venanzo (Perubia Province) and Cupaellow (Rieti prov-
ince) of Italy, by Gallo et al. (1984). The mafurites are constituted of kalsilite,
melilite and pyroxene.
Gittings et al. (1982) described pyroxenites containing an intergrowth of
nepheline and kalsilite from Batbjerg complex of Greenland. Kalsilite-bearing ul-
trapotassic rocks constitute a 350 km long belt trending NE
SW. It is extended
from the Derby Mountains of eastern Seward Peninsula to Kobuk-Selawik.
-
2.10 Analcite
The analcine problem and its impact on the geochemistry of ultrapotassic rocks
from Serbia has been discussed by Prelevic et al. (2002). According to them Ter-
tiary ultrapotassic volcanic rocks from Serbia occasionally display low levels of
K
2
OandK
2
O/Na
2
O. In these rocks, analcime regularly appears as pseudomorphs
after pre-existing leucite microphenocrysts. The process of leucite transformation in
Serbian ultrapotassic rocks is very thorough: fresh leucite survives only in ugan-
dites from the Koritnik lava
flows as well as in rare inclusions in clinopyroxene.
Their study focuses on the impact of
on the mineralogy and
geochemistry of the Serbian ultrapotassic rocks, using the samples where relecit
leucite has been used as a monitor for the process.
Some minettes from Bearpaw Mountains contain as much as 30 vol% of analcite
phenocrysts. They often occur as rounded clear grains (2 mm across), but in some
cases the crystals are turbid, and low in K
2
O content (<1 wt%, Table
2.9
), and the
total oxide content of analcite is 88
“
analcimization
”
89 wt% with 10 wt% H
2
O. The analcites in the
minettes are secondary formed by reaction between leucite and a Na-rich
-
fluid
(Macdonald et al. 1992).
This mineral occurs as an interstitial phase in the Square Top intrusion. Analcite
displays extensive Na, Al
Si substitution (Wilkinson and Hensel 1994). Their
composition extends from analcite to natrolite (slightly more Si-rich than
⃔
“
ideal
”
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