Náttúrufræðingurinn - 1985, Síða 59

Pedersen, A.K. & N. Hald. 1982. A
cummingtonite-porphyritic dacite with
amphibole-rich xenoliths from the
Tertiary central volcano at Króksfjörð-
ur, NW-Iceland. — Lithos 15: 137—
159.
Sveinn P. Jakobsson. 1985. íslenskar berg-
tegundir IV. Basaltískt íslandít og
íslandít. — Náttúrufræðingurinn 54:
77-84.
SUMMARY
Iceland rock types V
Dacite (rhyodacite)
by
Sveinn P. Jakobsson
lceland Museum of Natural History
P.O. Box 5320
125 Reykjavík
The article describes the rock type
dacite (rhyodacite). A Tertiary volcanic
plug in the Króksfjörður central volcano in
northwestern Iceland serves as an example
(cf. Pedersen and Hald 1982). The rock
contains about 36% by volume of pheno-
crysts, of which the following species have
been identified: plagioclase, quartz,
hornblende, cummingtonite, augite,
orthopyroxene, biotite, magnetite, ilme-
nite, apatite and zircon. The groundmass
is mainly composed of rhyolitic glass. A
chemical analysis of the Króksfjörður
dacite indicates a Si02 content of 69.35%.
It is tentatively proposed that the term
dacite (incorporating rhyodacite) should
be limited to rocks of the tholeiitic lineage
with a Si02 content of 67.5—72.0%, a
MgO content of 0.5 —1.5%, and with a
normative plagioclase composition
between An 20—30.
Dacite is comparatively rare in Iceland,
and has hitherto only been observed in 20
central volcanoes. Of these one is postgla-
cial, the Askja volcano in central Iceland.
It is estimated that some 1—3% of the total
production of volcanic rocks in Iceland
since Miocene is dacite.
153