Jökull - 01.12.1974, Blaðsíða 51

Fig. 6. Subglacial rhyo-
lite extrusion east of Kal-
manstunga near the base
of Strútur: pitchstone
lobes up to 10 m across
embedded in a granular
perlitic matrix. Grada-
tions between the matrix
and the pitchstone lobes
are seen.
Mynd 6. Líparitmyndun
i suðurhlið Strútsins frá
jökulskeiði (því áttunda),
sem gekk yfir í lok þriðju
liparitgoshrinunnar. Bik-
steinseitlar (= risabólstr-
ar) i perlusteinssalla.
The ignimbrite is overlain by up to 5 ande-
site lavas which bank against the slope of the
Reydarfell rhyolite succession. A rhyolite flow
is intercalated with them on the northern slope
of Bæjarfell, and between Bæjargil and Asgil
they are overlain by extensive rhyolite flows
which are thickest south of Reydarfell. The
andesites are present also in the basement of
Strútur and in Fljótstunguháls. In both places
porphyritic lava flows interfinger with them.
Tliis is the case also east of Selfjall. The rliyo-
lite plug of Iíleppar (Fljótstunguháls) and a
rliyolite flow exposed 1 km to the north of
Kleppar intrude and overlie the andesites. They
represent the northernmost outliers of rhyolitic
volcanism belonging to this phase.
The end of the thircl acid phase is marked
by a glacial horizon (no. 8 of Plate 1) which is
characterized by hyaloclastite volcanism of ba-
saltic (north side of Strútur basement). inter-
mediate (Kleppatagl, Selgil) and acid (Strútur,
Bæjargil, Asgil) compositions. Tillites and flu-
vial deposits generally interfinger with the
hyaloclastite piles. Characteristic of the sub-
glacially extruded acid rocks are large columnar
pitchstone bodies embedded in pitchstone
breccia and a granular perlitic matrix (Fig. 6).
Lavas which bank up against the hyaloclastites
are exposed on the north side of Strútur. They
are of reverse polarity like all members of the
third acid phase. In Strútur these lavas are
olivine tholeiites whicli reach a thickness of
more than 50 m. In Valagil at about 450 m
altitude a glacial horizon which is probably
equivalent to no. 8 is overlain by tholeiite lavas
which are also íound at the top of the south-
ernmost Giljahnúkur and in Skeidvallagil at
about 420 m alt. These tlioleiite flows are of
reverse polarity. They were probably banked
up against the clastic beds and hyaloclastites as
well.
Associated witlt the tliird acid phase is the
intrusion of acid and composite dykes and
cylindrical rhyolite plugs. A distinction between
intrusions belonging to the second and third acid
phase coulcl be made on the basis of the magne-
tic polarity, the third phase rocks being re-
versely magnetized whereas the second phase
rocks are normal. The majority of vertical ba-
saltic dykes occurring througliout the area also
are reversely magnetized. The acid intrusions of
the third phase occur in a 5—6 km broad NE—
SW trending zone between Ásgil and Kleppar
west of the presumed center of phase 2 (Fig. 7).
The central part of this zone including the
southern part of Fljótstunguháls, Tunga and
the mountain slopes south of Húsafell was sub-
ject to intense hydrothermal alteration which
continued throughout the third acid phase. The
alteration is strongest in the area where irre-
gular dykes and sheets are most abundant in-
volving such mineral phases as laumontite,
JÖKULL 24. ÁR 49