Jökull - 01.12.1982, Blaðsíða 76

TABLE 2. Major and trace element (ppm) com-
position of two gabbros (0 96 & 0 97) and
granophyres (0 111 & 0 112) from the Öræfi
district.
TAFLA 2. Efnagreiningar á fjórum bergsýnum úr Ör-
efasveit.
0 96 0 97 0 111 0 112
SÍÖ2 42.74 45.62 67.72 70.08
Tiö2 3.15 1.96 0.38 0.36
ai2o3 13.78 14.96 13.00 13.59
Fe2Ö3 7.24 4.01 3.03 3.26
Feö 8.75 7.06 1.47 1.50
Mnö 0.18 0.16 0.10 0.13
Mgö 6.95 9.35 0.25 0.12
Caö 12.36 13.30 2.59 1.01
Na20 2.25 2.00 5.00 5.94
k2o 0.33 0.31 3.34 3.16
P2O5 0.11 0.10 0.11 0.09
h2o+ 0.99 1.85 0.36 0.28
h2o_ 0.01 0.04 0.01 0.01
co2 0.00 0.17 1.25 0.48
Total 98.84 100.62 98.61 100.01
Rb < 10 < 10 65 56
Sr 322 305 116 97
Y 26 24 100 105
Zr 72 86 900 790
Cu 192 172 21 4
Zn 100 68 106 169
two samples (Table 2) show that the gabbro com-
position roughly corresponds to the basalts of the
Oræfi area in general.
The granophyre is medium to fine-grained. Sod-
ic plagioclase was the first phase to crystallise and
was succeeded by alkalifeldspar and quartz, which
in some places are found in graphic intergrowths
(see Prestvik 1979, p. 23). Accessories are olivine
(fayalite?), zircon as well as secondary calcite and
epidote. Compositionally (Table 2), the rock is
rather similar to the Öræfajökull rhyolites.
CHEMICAL CHARACTERISTICS
AND CLASSIFICATION
The major element and norm compositions of
representative samples of the Öræfajökull rocks are
presented in Table 3. Prestvik (1979) plotted all
analysed samples in alkali/silica and AFM dia-
grams and stated that the Öræfajökull rock series
was subalkaline with a typical tholeiitic trend. A
similar conclusion was reached at by Imsland (1978)
who plotted the unpublished data of Prestvik in the
classification diagrams of Irvine and Baragar (1979).
Jakobsson (1979) suggested that the rocks ofÖræfa-
jökull belonged to a transitional alkalic series charact-
erized by normative hypersthene. The basic rocks of
such a series are usually rather high in Fe and Ti,
and low in Al. The basalts plot above the division
line (Hawaiian division line) in the alkali/silica dia-
gram, but more evolved compositions tend to plot
below this line. The transitional series is supposed
to produce comenditic (i.e. peralkaline)rhyolite as
the most evolved rock type (Jakobsson 1979). Furth-
ermore, Oskarsson et al. (1982) referred to the rocks of
Öræfajökull as examples of a transitional alkalic/
tholeiitic series formed as a result of the “hybrid
divergent volcanism” of their dynamic model of rift
zone petrogenesis in Iceland.
Miyashiro (1978) divided alkalic rocks into two
main types: Those belonging to the Kennedy trend
and the Coombs trend respectively. The Coombs
trend is characterized by a hypersthene-normative
alkalic basalt which evolves into hawaiite, mugear-
ite, benmoreite, oversaturated trachyte and com-
endite by differentiation. The Coombs trend has
normative hypersthene and quartz just as a tholei-
itic series, and the two series may grade into each
other by increasing/decreasing Na,Ö+ K,(') cont-
ent. Furthermore, there seems to be no thermal
divide between them. In essence, it appears that the
transitional series of Jakobsson (1979) and the
Coombs trend of alkalic rocks (Miyashiro 1978) are
more or less identical in spite of the different names
assigned to the intermediate rocks by the two aut-
hors.
In the literature several systems or dividing lines
have been proposcd to discriminate between differ-
ent alkaline and subalkaline (here tholeiitic) rocks.
As can be seen in Fig. 3 all Öræfajökull rocks plot in
the subalkaline field if the dividing line odrvine and
Baragar (1971) is used. On the other hand, if one
uses the dividing line proposed by Miyashiro (1978),
the basic Öræfajökull rocks straddle the line bet-
ween the subalkaline and alkaline fields, and many
of the rocks in the basaltic andesite/andesite range
as well as all the rhyolites plot in the alkaline field.
None of the rhyolites, however, are comenditic as
required for silicic rocks of the transitional alkalic-
72 JÖKULL 32. ÁR