Jökull - 01.12.1978, Page 36

both for the vertical component of the mag-
netic field (A Z).
A total of 51 sun-compass oriented drill
samples (approx. 7 cm long, 2.5 cm diameter)
and 75 semi-oriented drill samples were taken
from the gabbro and the transition zone. The
semi-oriented samples were marked with
respect to up and down and the horizontal
magnetic north direction. In addition, 35
magnetic compass oriented handsamples were
taken, partly from the microgranites and
partly from the gabbro (see Fig. 2).
Sampling was accompanied by detailed
structural work, including measurement of
orientation of minor intrusions and joints. Age
relationship between the intrusions can be
seen for example in the western part of the
ravine at profile A (Fig. 3). Here gabbro brec-
cia is found in an acid intrusion, w hich again
is cut by basic dykes. At the western end of the
ravine in profile B (Fig. 3) gabbro is seen ov-
erlying microgranite with a contact dipping
ca 40° east, indicating a lens or sheet-like
shape for the gabbro.
Fig. 3 shows the A Z magnetic anomaly,
measured along 3 double profiles over the
Hvannadalur gabbro and the microgranite to
the west. Distances between the profiles were
too large to allow reasonably safe correlation
between the sets of double profiles. This is due
partly to variable topographic effects of the
gabbro outcrop and partly to inhomogen-
eities within the gabbro. Large topographic
effects are pronounced for example at profile
B located in a creek which cuts up to 10 m into
the gabbro with 30° slopes on either side.
Very large anomalous values of above 16000
gamma have been observed at profiles A
and Aa. The northeastern portion of profiles C
and Cc also exhibited a large positive anomaly
of 12 500 gamma. Magnetization determin-
ations on drilled samples from these areas
confirmed strong remanence intensities, in
part exceeding 40X 10-3emu/cc, whereas sus-
ceptibilities varied between 5 and 10X10-3
cgs units. From the anomaly map it thus
appears that the central and western portion
of the gabbro have strong remanent magnet-
Fig. 3. A Z magnetic anomaly map over the
Hvannadalur gabbro and adjacent micro-
granite (crosses), anomalies in gamma (arbi-
trary datum).
Mynd 3. Jafnsegulsviðskort (A Z) af
Hvannadalsgabbróinu og nœrliggjandi míkrógraníti
(krossar); mœlieining segulsviðsins er gamma.
ism with normal polarity. In contrast, some
eastern portions of the gabbro, the transition
zones, and the measured part of the micro-
granite are zones of minor magnetic anomalies
and magnetization. This situation has been
used to define locally a regional magnetic field
by determining the values over the westerly
microgranite. Measurements on oriented
samples from the microgranites and the tran-
sition zone proved the relatively low remanent
intensities (generally below 5 X 10-3emu/cc)
and low susceptibilities of about 1 — 3X 10~3
cgs units for the microgranites.
34 JÖKULL 28. ÁR