Jökull - 01.12.1978, Blaðsíða 38

Fig. 5. A Z anomaly over the southern por-
tion of Geitafell gabbro and adjacent Hof-
fellsjökull sandur, anomalies in gamma (arbi-
trary datum).
Mynd 5. Jafnsegulsviðskort (A Z) af suðurhluta
Geitafells og sandinum sunnan gabbróinnskotsins;
mœlieining segulsviðsins er gamma.
east. Annels (1967) and Newman (1967) desc-
ribed this gabbro and outlined its boundaries.
Magnetic AZ profiles (a-o, Fig. 5) have been
measured over the exposed gabbro and the
sandur to the west and south in order to define
the southerly extension of the gabbro below
the sandur.
The magnetic measurements, supple-
mented by some vertical gradient determin-
ations, show the bulk of the Geitafell gabbro
to be reversely magnetized, partly with strong
intensities. Detailed drilling was carried out
across the strongly magnetized zone along the
ca. 1 km long profile D and also profile E,
which extends into the plateau basalt area
(Fig. 4). In all 139 sun compass oriented and
98 semi-oriented drill samples have been
taken from every dyke and sheet along the two
profiles, with a minimum of three oriented
samples at each sampling site. About 25 mag-
netically oriented handsamples have also been
taken from the gabbro for combined
geochemical and magnetomineralogical ana-
lysis.
The orientation and age relationships of the
traversed dykes have been investigated.
Relative ages can be summarized: An old sys-
tem of basic dykes has been cut by acid dykes
containing angular fragments of the gabbro.
The acid dykes were subsequently cut by two
generations of basic dykes. Some of the latter
have a doleritic appearance and probably fall
under Annels’ (1967) term “brown doleritic
dykes”. A still younger generation of basic
dykes with porphyritic texture is discernible at
a few places, for example outside the gabbro
proper in the Midfellsgil canyon, where
sampling has also been undertaken.
The northern portion of Geitafell gabbro
has not been sampled, but sampling is plan-
ned.
Fig. 5 shows the location of magnetic profi-
les and the anomaly pattern found. An ex-
tremely strong negative AZ anomaly over the
southern tip of the gabbro outcrop is the
dominant feature of the map. The anomaly
extends approximately 500 m to the southeast
of the outcrop. In a northerly direction, the
negative anomaly diminishes in amplitude,
and correlation between profiles is difficult.
Topographic effects become very strong and
allow only tentative contouring. The magnetic
measurements in their topographic setting
show that by and large the investigated gab-
bro has a reverse magnetization, locally very
strong. This means that the induced magneti-
zation must be small compared to the
remanent magnetization. This was confirmed
by laboratory measurements on the samples
drilled. The paleomagnetic results necessary
to aid in the structural interpretation are
presented in Fig. 6, whereas the magneto-
mineralogical investigations will be dealt with
elsewhere (Schönharting, in prep.).
Nearly all the gabbro samples exhibit
36 JÖKULL 28. ÁR