Jökull - 01.12.1978, Síða 37

Stable components of remanence, isolated
by AF demagnetization, were found to have
declinations between 20° and 100° east, aver-
aging 70° east for the bulk of the gabbro
and transition zone samples, as well as for
most microgranites. This, together with
slightly too low inclinations, can most
easily be interpreted as due to a tilting of
the gabbro and associated intrusions
by 35° towards NW after emplacement
or, more accurately, after cooling below
the respective Curie temperatures of ca. 550 to
580°C. An exception is provided by samples
from the eastern part of profile A, where
northwesterly declinations of magnetization
have been found. This could be interpreted
either by a different tectonic movement at this
locality or, alternatively, emplacement of this
gabbro portion after the main tilting phase at
a time of slightly anomalous geomagnetic
field. The disturbing effect of the gabbro’s
own demagnetizing field during cooling must
also be taken into consideration, and locally,
close to zones of strong magnetization,
declinations and inclinations can be expected
to be widely scattered.
The northwesterly tilt of the bulk of the
gabbro and microgranites conforms with the
dip of plateau basalts within the flexure zone
of SE Iceland. Walker (1975) pointed out the
relationship between gabbro and sheet
emplacement and the subsidence of the
basalts, which is strongest in the immediate
vicinity of the intrusions. The implications of
this with regard to the timing of the tilting will
be discussed in the last section of this article.
In some microgranite samples reversed
remanent magnetization has been found.
Apparently, there exist different units of
microgranite which are possibly related to
different stages of acid intrusions. Sufficient
sampling has not been carried out to delineate
unit boundaries.
Model calculations for the anomalies on
profiles C and Cc suggest a sheet-like gabbro
body, dipping about 60° towards NE. This is
not in contradiction to the 40°dip towards
east observed for the lower boundary of the
gabbro at profile B, in view of the irregular
form of the gabbro intrusion. This, i.e. the
varying width of the gabbro outcrop and its
small thickness of less than 200 m, indicates
that the gabbro does not represent the upper-
most part of a larger magma chamber. More
sampling and magnetic measurements are
planned to further delineate structural
relationships between the gabbro and the acid
intrusions.
THE GEITAFELL GABBRO
The Geitafell gabbro (or more correctly the
Geitafellsbjörg gabbro — Jónsson, 1954) is
accessible from the south along the sandur-
plain to the east of the Austurfljót river. The
gabbro is exposed over an area of about 2 X 0.5
km. In contrast to the Hvannadalur gabbro, it
is cut by numerous dykes and sheets (Fig. 4)
which mask the contact between the gabbro
and the plateau basalts to the southeast and
Fig. 4. Geological sketch map of the
southern portion of Geitafell gabbro with
sampling profiles D and E.
Mynd 4. Einfaldad jarðfrœðikort af suðurhluta
gabbróinnskotsins í Geitafelli með sýnatökulínum D
ogE.
JÖKULL 28. ÁR 35