Jökull - 01.12.1978, Page 41

zones with acid intrusions of about same age,
however, deserve further discussion.
No field evidence has been found to suggest
that the zones of strong magnetization are
related to feeder channels, or alternatively to a
layer within a magma chamber where the
heavy iron oxides accumulated during the
first stages of magma cooling. The proximity
of acid intrusions around both the Hvanna-
dalur and Geitafell gabbros, ranging from
massive microgranites to mm-thick veins, and
occasionally signs of magma mixing on a
limited scale, lends support to the hypothésis
that the variation of oxygen fugacities in the
gabbroic magma might be the primary cause
for the variation in the magnetization. The
total Fe content in the gabbro samples
appears from limited chemical analysis to be
more or less constant, but iron oxide enrich-
ment has been found in polished sections from
the zones of strong magnetization.
The observed proportion of the strongly
magnetized zones to the whole gabbro
volume, 15% and 30% at Geitafell and
Hvannadalur localities respectively, is prob-
ably not typical for gabbros at large depths,
where more homogeneous conditions can be
expected. The sheet-like shape of the inves-
tigated gabbros with a strike direction almost
perpendicular to the eastern rift zone in Ice-
land makes it somewhat unlikely that these
gabbros are typical products of a spreading
process. On the basis of the present field ob-
servations it is not possible to enter into a
discussion about the mechanism of sheet
emplacement. It appears possible, however,
that an interpretation of the gabbros in terms
of intrusions along minor transverse faults
within an old rift zone (= flexure zone of SE
Iceland) might explain the shallow position of
the gabbros and their strike direction.
According to Annels (1967) the gabbros were
emplaced ca. 0.5 to 1 km below the surface at
the time.
Due to the presence of a number of gabbros
and numerous sheets within that part of the flex-
ure zone where the plateau basalts have been
strongest tilted, Walker (1975) suggested that
the flexure formed by subsidence caused by
the density increase of a solidifying basaltic
magma during gabbro emplacement. The
Hvannadalur intrusion has been radiometri-
cally dated at around 2 M.y. and acid dykes
4.5 km from Geitafell gabbro at around 5 and
7 M.y. (Gale et al. 1966). Local isostatic adjust-
ment resulting in a flexure zone less than 10
km wide which formed within a few million
years time or less, together with very rapid
tilting towards the intrusion centres, requires
a substratum of very low viscosity. It is pro-
posed here that the substratum was liquid
magma in a magma chamber or a chain of
magma chambers below the axis of the flexure
zone. The solidified part of a magma chamber
is thus expected to subside until approximate
mass equilibrium is reached. The exposed
gabbros within the flexure zone are probably
too small to represent the solidified magma
chambers but are perhaps some 100 m to 1000
m higher up. The solidified magma chambers
constitute perhaps some of the magnetic
sources required to form the anomaly patterns
in zones of regional extension.
The dykes and sheets cutting the gabbros
and within the alteration zones around the
gabbros (Annels 1967) have génerally only
minor magnetization intensities, between 0.5
and 5X 10-3emu/cc. This can be related
directly to chemical alteration and oxidation
of the magnetic ore components at moderate
temperatures. Magnetic anomalies from these
dykes and sheets are therefore only of minor
importance in the areas investigated.
More field work is needed to clarify the
structural situation of the investigated gab-
bros. Magnetomineralogical investigations
are being conducted to clarify the oxidation
conditions during crystallization and cooling
of the magnetic ore components.
ACKNOWLEDGEMENTS
Support from Icelandic authorities, namely the
Icelandic Research Council, the National Energy
Authority and from the local police station in Höfn,
chief Elias Jonsson, is gratefully acknowledged.
JÖKULL 28. ÁR 39