Jökull - 01.12.1980, Side 40

60
REVERSED
REVERSED
40-
NORMAL
20
(dykes)
(lavas)
20-
H
Fig. 3. Distribution of iron sulphides (pyrite
and pyrrhotite) as a percentage of the whole
rock area in normal and reversed dykes.
Fig. 2. Distribution of the oxidation states
(M) in the normal (lower histogram) and
reversed (upper histogram) dykes and lavas
from Reydarfjördur where the vertical axis
refers to the percentage of the number of
samples possessing the given polarity. The
data for the lavas are based on the distribution
of oxidation states in 23 reversed and 15 nor-
mal lavas from Reydarfjördur from a less
rigorous analysis of Fowler (1975); the shaded
area of the lava histograms represents the
percentages of lavas with double Curie points.
Mytid 2. Oxunarstig í rétt (neðra súlurit) og öfugt
segulmögnuðum göngum og hraunum við Reyðarfjörð.
Skyggð1 hluti súlanna á við sýni með tvö Curie-hita-
stig.
flows. These lavas tend to have relatively low
oxidation states by comparison with many
lava successions studied elsewhere, but also
exhibit some higher oxidation classes not ob-
served in feeder dykes from the same area (Fig.
2). This is in accord with Ade-Hall and Lawley
(1970) who find that the degree of deuteric
oxidation tends to be less in basaltic dykes
than in lavas.
When the distribution of deuteric oxidation
states is plotted as a function of polarity (Fig.
2) it is observed that the reversed dykes are
characterised by higher deuteric oxidation
states than the normal dykes. This same cor-
relation was noted in Scottish Tertiary dykes
(Ade-Hall and Wilson 1969, ca. 55 m.y. old) and
Greenlandic Jurassic dykes (Piper 1975, ca. 168
m.y. old), and implies that this correlation
cannot be explained in terms of external
(atmospheric) effects.
Mynd 3. Magn járnsúlfíða í rétt og öfugt segul-
mögnuðum gangsýnum (%)■
Only for the sulphide content was any dif-
ference noted in the quantity of opaque phase
present in each polarity group; both as a per-
centage of total opaque content and whole
rock volume sulphides are about twice as
abundant in dykes of normal magnetisation
(Fig. 3) compared with dykes of reverse mag-
netisation. This is in accord with the oxidation
correlation since it is to be expected that
sulphide content will show an inverse
relationship to oxidation state. However this
simple correlation is complicated at Reydar-
fjördur by the presence of hydrothermal
sulphides which appear both in the ground
mass and in vesicles; in this context it is note-
worthy that the average sulphide content is
higher in the range of dykes 29—58 (Table 1)
which coincide with the pyropylitised zone of
the Reydarfjördur silicic centre (Gibson et al.
1966, Fig. 24).
The samples with least deuteric oxidation
are most susceptible to granulation (a corre-
sulphides
Fig. 4. The distribution of average .granu-
lation index (G), Maghemitisation index
(Mh), and sulphide percentage of total rock
volume as a function of oxidation class (M).
Mynd 4. Dreifing ummyndunarstuðlanna G og Mh,
svo og súlfíðinnihalds, með hakkandi oxunarstuðli.
38 JÖKULL 30. ÁR