Jökull - 01.12.1970, Blaðsíða 71

foss, 10 m south of Ás). The relatively high
base level (50—60 m) south of Gíslholtsvötn
may be caused by faults (actually seen near
Steinslaekur) but this level more likely reflects
the uneven topography at the time of emplace-
ment of the young lavas.
Thus, the lavas to the north of Thjórsá seern
to have solidified in valleys with a southerly
trend, but spread over an erosional plain
further south. The lavas in the north piled up
against the slopes of the valleys and attained
a thickness of 80 m at least. The sediments
below the lava flows indicate the presence of
streams flowing along the bottoms of the valleys
whereas the hillslopes were covered by moraine.
The greatest depression probably occupied
by a major river depositing sand and gravel,
lay in the eastern part of the area now covered
by the lavas. To the west the relief may have
looked similar as today (i.e. a typical glacial
relief) except that the hills were more elongat-
ed in NE—SW-direction according to the strike
of the rocks. Small lakes existed in the old
relief that gave rise to hyaloclastite and pillow
lava formation discussed below.
PETROGRAPHY OF THE
INTERGLACIAL LAVA FLOWS
Petrographically the lavas fall into two di-
stinct groups that occur in separate areas. To
the south of Thjórsá the rock is an olivine
basalt whereas to the north of the river the
lavas are tholeiites. These are field terms wide-
ly used in Iceland in recent years. More correct
designation would probably be olivine tholei-
ite instead of olivine basalt. The inineral com-
postion of the olivine basalt (Plate I, A) found
by using a point counter is shown in Table 1.
Plagioclase occurs in the groundmass as lath
shaped polysynthetic twins ranging in size from
0,05—0,6 mm. Zoning was observed in a few
tabular crystals dispersed throughout the
groundmass. A determination of refractive in-
dex of the plagioclase gave ny = 1.570 ± 0.002
and na = 1.562 ± 0.002 corresponding to 65%
An. Pyroxene is found as faintly coloured grains
smaller than the plagoiclase showing intergra-
nular relation to it. It seldom exceeds 0.3 mm
in size. An average of 6 measurements of 2 V
gave 48° which corresponds to augitic composi-
TABLE 1
Plagioclase 1. • • • 49,2% 2. 48,6%
Pyroxene • • • 32,4% 34,2%
Olivine • • • 8,6% 8,0%
Magnetite • • • 9,0% 9,2%
Alteration products .... ■ • • 0,8% -
1. =. Thinsection nr. 903. Sample from west of
Lýtingsstadir (831 points).
2. = Thinsection nr. 907. Sample from Rangá
south of Geldingalaekur (845 points).
tion. Olivine forms equant, sometimes corroded
and slightly altered grains similar in size to
pyroxene. These two minerals are very similar
and it is often difficult to distinguish between
them. The olivine however is clearer and less
coloured than the pyroxene. Besides the char-
acteristic irregular cracks, the olivine has often
a distinct cleavage parallel (010) indicating a
fairly iron rich variety. Ore forms interstitial
skeletal, needleshaped or ragged grains having
about the size of pyroxene. It is probably mostly
magnetic. Olivine is sometimes slithly alter-
ed to yellowish brown (non pleochroic) idding-
site. Plagioclase shows an incipient alteration
to greenish or yellow montmorillonite. Cavities
are sometimes lined with a thin layer of mont-
morillonite.
The mineral composition of the tholeiite
(Plate I, B) is shown in Table 2, as revealed
by point counting of thinsections from tlie re-
latively well crystallized colonnade rocks.
Under the microscope tlie tholeiite is seen to
be a microporphyritic rock containing about
13% microphenocrysts of plagioclase, pyroxene
and subordinate olivine, partly grouped to-
gether as glomerophenocrysts, seldom more
than 1 mm in size. Plagioclase micropheno-
crysts form lath shaped polysynthetic twins
usually zoned up to 1 mm long. Pyroxene
microphenocrysts, which may occur in ophitic
relation to plagioclase, are up to 0.4 mm in
size and often show hourglass structure. Olivine,
which only occurs as microphenocrysts, forms
heavily corroded crystals about the same size as
pyroxene. It is usually somewhat altered to idd-
ingsite. The groundmass of the tholeiites has
intergranular texture and is more finegrained
JÖKULL 20. ÁR 69