Jökull - 01.12.1970, Page 72

TABLE2
1. 2. 3.
Plagioclase . .. . .. . 39,6% 36,8% 37,8%
Pyroxene ... 42,2% 42,8% 45,4%
Olivine 1,2% 0,8% 1.1%
Magnetite ... 12,3% 10,0% 9,9%
Glass 4,7% 9,6% 5,8%
1. Thinsection no. 927. Rock from lower part
of flow in Núpstúnskista (846 points).
2. Thinsection no. 924. Rock from lower part
of flow in Skardsfjall above the farm Skard
(842 points).
3. Thinsection no. 988. Colonnade rock from
lower part of flow in Thjórsárholt (853
points).
than in the olivine basalts. Plagioclase forms
unzoned twinned laths only 0.002—0.1 mm in
size. The bulk composition of plagioclase laths
and microphenocrysts near rims was found by
measurement of refractive index ny = 1.567
± 0.002 and na = 1.558 ± 0.002 corresponding
to about An 60%. Pyroxene forms grains only
0.001—0.05 mm in size. Measurement of 2V of
the pyroxene showed the presence of both pige-
onite with very low optic axial angles and
augites of very much higher optic axial angles
(2V average of 5 measurements = 50°). Magne-
tite usually forms equidimensional grains dis-
tinctly less ragged than in the case of the
olivine basalts, although needleshape may also
occur. A brownish glassy residum fills sniall
interstices between crystals.
Rock from the upper part of the flows later
referred to as the entablature is seen under the
microscope to consist of an extremly finegrain-
ed glassy groundmass (Plate I, C) and micro-
phenocrysts that are the same as in the coarser
colonnades. It is impossible to state accurately
the amount of glass in the thin sections but a
value somewhere near 20—30% for the brown
translucent glass cannot be very far wrong. An
addtional 10—15% opaque glassy residue is
crowded around the ore grains obliterating
their margins.
FIELD CHARACTERISTICS
OF THE INTERGLACIAL
LAVA FLOWS
In hand specimen the olivine basalt is a
rather coarse grained rock without phenocrysts,
gray coloured when fresh but sometimes show-
ing incipient yellowish alteration. The tholei-
ite is a fresh looking gray coloured rock that
is fine grained, dense, and distinctly flow band-
ed except in the glassier entablatures that are
dark and flinty. Vesicles in both rock types are
devoid of secondary minerals.
Columnar jointing is most conspicuous in
the tholeiites north of Thjórsá and in the
northern part of the olivine basalt area. Well
developed columns occur in Hólahnúkar where
the lavas occupy a depression in the old base-
ment with long, curved columns fanning up-
wards from the sides and bottom (Fig. 5). In
this area conditions were given for solidifica-
tion under static conditions when the lavas
were ponded into valleys and depressions. At
several localities in Hreppar the lavas consist
of a lower regularly columnar portion and a
hackly jointed upper portion (Fig. 6 and 7).
In Galtafell five such flows are seen in the
EXPLANATION TO PLATE I
A Olivine basalt showing intergranular intergrowth of olivine, augite, plagioclase and ore.
Ásmundarstadir, interglacial lava flow, thin section no. 906. Magn. x 100. Ordinary light.
B Tholeiite showing a glomerophenocryst of ophitic plagioclase and pyroxene set in a fine-
grained intergranular groundmass. Note euhedral shape of magnetite. Skardsfjall, colonnade
of an interglacial lava flow, thinsection no. 986. Magnification X 100. Ordinary light.
C Tholeiite showing glomerophenocrysts as in B set in an extremely fine grained and glassy
matrix. Skardsfjall, entablature of same lava flow as B, thinsection no. 987. Magnification
Xl00. Ordinary light.
Myndir af þunnsneiðum úr ungu hraunum, í ca. hundraðsfaldri stœkkun,
A fremur grófkornað ólivínbasalt. B ólivinfátcekt basalt úr stuðlabergi í Skarðsfjalli, C sama, en
úr kubbabergi, mun fínkornaðra og glerkennara.
70 JÖKULL 20. ÁR