Náttúrufræðingurinn - 1963, Blaðsíða 27

NÁTTÚRUFRÆÐINGURINN
21
SUMMARY
On inclusions in Icelandic Rocks
(Preliminary Report)
by Jón Jónsson
State Electricity Authority, Reykjavík.
The object o£ this report is to draw attention to the existence of coarse
grained inclusions of foreign origin found in the fine grained Icelandic vol-
canics. Xenoliths are previously known from several localities in Iceland. They
liave mostly been found in pyroclastic rocks. Here the author gives a brief
description of inclusions found in lavas in 18 different localities in Iceland.
(For the principal localities see the map fig. 1). The inclusions are light
coloured contrasting witli the usually dark or black basaltic material. They
contain plagioclase, clinopyroxene and/or olivine. A number of inclusions
liave been examined in thin sections but a detailed minerological study is
in preparation. The inclusions are found in basaltic lavas of late Tertiary
age, interglacial doleritic lavas, palagonite tuffs and in recent lavas. They
liave, however, not been found so far in the older Tertiary flood basalts.
Near Reykjavik there are at least 5 different lava flows named Hólms-
hraun (hraun = lava flow), which are according to a C14 dating less tlian
5300 ± 340 years old. In one of these flows, Hólmshraun II the second oldest,
inclusions are founcl in great number. Average diameter is 3—5 cm, but in-
clusions up to 10 cm have been observed.
Most of the inclusions in Hólmshraun II are vesicular just as the lava
itself. Tlie largest inclusions show, however, a compact core witli gabbroic.
appearance. Principal mineral constituents are plagioclase, clinopyroxene and
olivine cf. Table I 4. For comparison the mineral composition of the Hólms-
hraun II lava: Plagioclase 44,5, Pyroxene 43,2, Olivine 7,6 and opaqc material
4,7. To the unaided eye tlie contacts between dark lava and light inclusion
mostly are sharply defined, under the microscope, however, it is more or less
gradational. At the contact plagioclase crystals frequently are heavily corroded,
indicating nonequilibrium with the surrounding melt. The transition zones
between lavas and inclusion seern to indicate the start of thermal breakdown
of the rock. In an inclusion specimen from Hólmshraun II, refractive indices
of plagioclase cleavage fragments were determined in immersion liquids. nDZ
ranges frorn 1,570 to 1,576, An65—An77 t.c. labradorite and bytonite.
In phenocrysts of the lava itself nDZ ranges from 1,566 to 1,569. It is to
bc stressed, however, that so far only few measurements liave been done.
The plagioclase of the same inclusion is polysyntetically twinned, but zon-
ing has not been observed. In the plagioclase phenocrysts o£ the lava zoning
is frequently observed. Hour glass structure is common in the pyroxene pheno-
crysts of the lava but has not been observed in the inclusions. All the inclusions
hitherto found have nearly the same composition (Table I).