Náttúrufræðingurinn - 1985, Page 38

SUMMARY
Icelandic rock types IV.
Basaltic icelandite and
icelandite.
by
Sveinn P. Jakobsson
Icelandic Museum of Natural History
P.O. Box 5320
125 Reykjavík
The article describes the tholeiitic inter-
mediate rock types basaltic icelandite and
icelandite. A Tertiary lava flow in the
Þingmúli (Thingmuli) central volcano in
eastern Iceland serves as an example for a
basaltic icelandite. This is a very fine-
grained and dense rock (Fig. 1A) with few
phenocrysts (less than 1 percent) of
plagioclase, magnetite, augite and apatite.
The groundmass consists of plagioclase,
clinopyroxene and ore. A chemical analys-
is (Table I) indicates a Si02 content of
55.03 percent and a relatively high MgO-
content.
A postglacial lava at Lúdent in the
Námafjall volcanic system in northern Ice-
land (Fig. 2) serves as an example for
icelandite. The lava is heterogenous with
Si02 varying between 56.2 percent and
66.8%, a variation which may be typical
for the intermediate eruption units. The
sample chosen has a Si02 content of 58.9
percent and MgO content of 3.2 percent
(Table II).
It is extremely fine-grained, whith about
1—2 percent of phenocrysts of plagioclase,
augite, magnetite, hypersthene and olivine
(Fig. 1B). The term icelandite was intro-
duced by Carmichael (1964), in order to
avoid confusion with the typical orogenic
andesites.
Basaltic icelandite and icelandite are not
common rocks in Iceland, they have, how-
ever, been discovered in most of those
tholeiitic central volcanoes which have
been carefully studied (Fig. 3). By rough
estimate, some 5 percent of all volcanic
formations in Iceland may be intermediate
tholeiitic rocks.
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