Jökull - 01.12.1983, Síða 68

TABLE 2. C.I.P.W. Norms* of Analysed Lavas and Tephra.
TAFLA 2. Reiknuð steindasamsetning í sýnum úr Tóflu 1.
S4 S5 S7 S8 S12 S20 S21 S30 S33 S39
Qz 2.8 — _ — — — 0.3 — — 19.1
Or 3.6 4.0 4.0 7.5 2.0 5.3 5.0 6.0 4.9 21.1
Ab 25.0 26.3 26.5 37.2 10.7 22.7 30.3 31.5 27.8 48.6
An 23.5 26.9 27.2 16.9 16.1 21.6 17.8 17.4 21.1 3.8
Ne - - - 0.6 - - - - - —
Di 19.5 20.9 21.3 18.9 30.6 21.2 23.6 20.7 21.9 2.9
Hy 12.8 10.5 11.4 - 7.3 12.9 10.7 9.4 9.0 2.0
Ol - 1.7 0.4 12.7 28.4 7.8 - 4.8 5.2 —
Mt 5.7 3.8 4.7 1.4 2.7 3.8 5.2 3.0 3.3 2.0
Ilm 6.1 5.2 4.0 4.5 1.8 3.9 6.5 5.8 5.6 0.5
Ap 1.0 0.8 0.7 0.2 0.3 0.7 0.5 1.5 1.2 0.1
An/An+Ab 48.4 50.6 50.6 31.2 60.1 48.8 37.0 35.5 43.2 7.2
Mg/Mg+Fe2+ 66.2 65.2 65.4 50.0 85.1 79.3 63.2 51.6 59.7 24.2
Diff. Ind. 31.4 30.3 30.4 45.4 12.7 28.1 35.6 37.5 32.7 88.8
* Expressed in % cation equivalents and based on analyses recalculated to 100% water-free and with the Fe203/Fe0
ratio adjusted where necessary to an upper limit of Fe203 fixed at %Fe203 = %Ti02+1.5, according to the
procedure outlined by Irvine and Baragar (1971).
palagonite tuff-breccias and associated cinder cones
were briefly observed and sampled to the north-
west on Fimmvörðuháls on the eastern flank of the
Eyjafjöll volcanic complex. In addition a record of
Postglacial volcanic activity emanating from the
nearby volcanic centres, especially Katla as de-
monstrated by Einarsson et al. (1980), was observed
in loessic soil profiles throughout the Sólheimajök-
ull area.
THE GEOCHEMISTRY OF THE
VOLCANIC ROCKS
The chemical analyses of selected volcanic rock
samples are listed in Table 1 and the corresponding
C.I.P.W. norms in Table 2. Further detailed in-
formation (petrography, locality, name, etc.) on
each rock sample is given in Table 3.
These analyses may indicate that there have been
changes in the chemistry ofthe magma type erupted
over the period of time from the eruption of the
oldest rocks in the area (Unit A) to the most recent
rocks in the Fimmvörðuháls area. We assume that
this period of time was something of the order of two
million years. The dominant basalt lava type would
appear to have changed from a tholeiitic type in the
oldest rocks (Unit A and much of Unit B), to a
transitional alkalic type (throughout most of the
volcanic succession) with appredably higherNa,O,
K20 and TiO, contents and a higher Fe/Mg ratio,
to a nepheline-normative alkali olivine basalt type
in the most recent volcanic activity in the col area of
Fimmvörðuháls. This variation is illustrated on
Figure 2 which is a plot of wt.% Na^O + K20
against wt.% SiO, for all analysed lava samples.
Sample S12, the lava from Unit A at the foot of
Jökulhaus North, is characterised by both high
modal and normative olivine contents. It is also
distinctly hypersthene normativeand lies well with-
in the recognised sub-alkaline i.e. tholeiitic field
(MacDonald 1968; Irvine and Baragar 1971) on the
alkalies: silica plot. Certainly its lower TiO>, P205,
Na20, K20 and Ah Oj contents and lower Fe/Mg
ratio contrast with the subsequently erupted volc-
anic rocks. On the basis of its chemistry and norma-
tive mineralogy it might seem appropriate to classi-
fy the rock as a picritic olivine tholeiite but there ís
little doubt that it is comparable to other rocks
related to the Eyjafjöll volcanic system customarily
referred to as ankaramites (Steinþórsson 1964\Jakobs-
son 1979). Whilst these authors have indicated that
certain of these rocks may be intrusive and also
cumulative, the intimate associadon in this instance
with related palagonitized brecda indicates an ex-
66 JÖKULL 33. ÁR