Jökull - 01.12.1978, Page 35

e.g. Ade-Hall et al. (1975), Bleil and Petersen
(1977), indicate a model which is more com-
plicated than that of an only 500 m thick layer
2a with blocks of alternating magnetic
polarities (Talwani et al. 1971). Polarity rever-
sals in single drillholes, the occurrence of stable
non-dipole magnetic inclinations and too
small averaged “effective” magnetizations
suggest that deeper levels of layer 2 and per-
haps 3 must be included in the “magnetic
layer” (Angenheister et al. 1977).
The relative increase of intrusives with
depth, their generally higher Curie points and
stronger resistance to chemical alteration due
to their compactness compared to basalts, en-
hance the importance of basic intrusions as
magnetic sources at depth.
The lower boundary of the magnetic layer
in Iceland has been postulated by Schönharting
(1969), Kristjansson (1972) and Kristjansson and
Watkins (1977) to be at more than 2 km and
perhaps even 5 km depth. Therefore it
appeared reasonable to investigate mapped
intrusives in SE Iceland by field magnetic
methods, to select those suitable for con-
tributing to the magnetic layer, and to study
their structure, composition and magneti-
zation in detail. The gabbro intrusions at
Hvannadalur and Geitafell were chosen be-
cause of relatively easy access and good expo-
sures.
THE HVANNADALUR INTRUSIONS
The basalts dip about 20° towards NW in
Steinadalur at Vindás where Kaldakvísl cuts
through the plateau basalts. Entering
Hvannadalur from the south, the plateau
basalts can be seen to be gradually replaced by
dykes and sheets until the acid and basic in-
trusions of Hvannadalur are reached, The
central gabbro is on the western side of the
river Dalsá at about 200 m altitude (Annels,
1967). Its exposure is elongated,
approximately 2 km long, slightly arcuate and
is oriented roughly N—S. The gabbro is sur-
rounded by acid intrusions, exposed on the
westem side of the valley as roches mouton-
Fig. 2. Geological map of Hvannadalur
with magnetic profiles (dotted), detailed dril-
ling profiles (thick dashes) and handsample
sites (circles).
Mynd 2. Einfaldað jarðfrœðikort af Hvannadal.
Segulmœlilínur eru syndar með punktalínum, söfn-
unarstaðir kjarnaborsýna með slilinni línu og söfn-
unarstaðir handsýna með opnum hringjum.
nées and in contact with the gabbro in creek
profiles. The term microgranite has been used
in the field to cover the various acid intrusions.
It is possible that the “hybrid gabbro” of
Annels (1967), a transition zone between the
gabbro and the acid intrusions, was
occasionally identified as microgranite, when
field observations were not sufficient.
Magnetic reconnaissance measurements
were followed by detailed sampling along
profiles where strong anomalous magnetic
fields had been found. Station spacing along
the magnetic profiles varied from 1 to 10 m,
with an average of 5 m. Magnetometers used
were a “Minimag” and an “Askania GfZ”,
JÖKULL 28. ÁR 33