Jökull - 01.12.1982, Blaðsíða 77

TABLE 3. Major element and CIPW norm composition of representative rocks from Öræfajökull.
Before norm calculations the I'ejO j/FcO ratios for theoleiites, theoleiitic icelandites and icelandites were
adjusted to 0.15, and all analyses were recalculated to a 100% water-free basis.
TAFLA 3. Efnagreiningar og reiknuð steindasamsetning bergsýna frá Örrefafókli.
0 166 0 184 0 67 0 228 0 248 0 267 0 136 0 98 0 200 0 135 0 50 0 261
sío2 48.64 48.99 48.38 50.77 50.57 51.14 55.06 57.80 63.36 65.76 70.60 68.35
TiOs 2.21 2.81 2.80 3.17 3.06 2.98 2.24 1.62 0.62 0.52 0.15 0.22
ai2o3 14.38 14.00 13.60 13.14 14.37 14.31 13.90 13.73 14.37 14.44 13.35 13.02
Fe203 3.53 3.22 3.23 4.80 3.14 4.43 1.85 2.10 1-.88 1.28 0.66 0.83
FeO 8.45 9.98 11.77 9.84 10.25 8.15 10.14 10.61 4.83 4.22 1.75 2.07
MnO 0.19 0.21 0.24 0.22 0.24 0.24 0.23 0.32 0.20 0.16 0.07 0.09
MgO 7.35 6.25 5.10 5.40 4.25 3.44 2.58 1.56 0.45 0.40 0.10 0.03
CaO 11.71 9.87 10.03 8.84 7.96 7.78 6.62 5.45 2.95 2.53 1.58 0.85
Na20 2.35 2.68 3.21 2.90 3.65 3.75 4.26 4.88 6.13 5.62 5.00 4.25
k2o 0.25 0.41 0.57 0.67 1.31 1.42 1.74 1.53 2.34 3.32 4.02 4.49
p2o5 0.25 0.35 0.27 0.39 0.57 0.79 0.87 0.58 0.17 0.08 0.00 0.02
h2o+ 0.83 1.05 0.25* 0.48 0.12 0.78 0.21* 0.21* 1.81 0.34* 0.48* 3.94
h2o- 0.08 0.10 - 0.13 0.04 0.08 - - 0.04 - — 0.04
Total 100.22 99.92 99.45 100.75 99.53 99.29 99.69 100.39 99.15 98.67 97.76 98.20
Q _ 0.11 — 2.33 — 1.37 4.34 6.20 11.40 13.66 23.31 25.26
Or 1.49 2.46 3.40 3.97 7.80 8.55 10.34 9.02 14.21 19.95 24.42 28.36
Ab 20.06 23.00 27.42 24.58 31.13 32.33 36.25 41.20 53.31 48.36 43.49 37.73
An 28.20 25.31 21.21 20.89 19.10 18.36 13.74 11.01 4.92 4.44 2.17 3.61
Di 23.61 18.21 22.80 17.07 14.19 13.21 11.53 10.54 7.83 6.79 5.18 0.71
Hy 14.99 22.20 7.11 21.54 15.98 16.20 15.24 15.10 3.93 3.71 0.16 -
Ol 4.59 - 9.24 - 2.11 - - - - - - -
11 4.24 5.41 5.37 6.03 5.86 5.77 4.28 3.07 1.21 1.00 0.29 0.44
Mt 2.24 2.49 2.83 2.70 2.51 2.35 2.26 2.52 2.80 1.89 0.98 1.39
Ap 0.60 0.84 0.65 0.93 1.36 1.91 2.07 1.37 0.41 0.19 — 0.05
* water determined as total H20.
0 166 Tholeiite, Slaga
0 184 Tholeiite, Svinafell
0 67 Tholeiite, Kvisker
0 228 Tholeiite, Skaftafellsheidi
0 248 Tholeiitic icelandite, Hvannadalskambur
0 267 Tholeiitic icelandite, Breidamerkurfjall
0 136 Icelandite, Svinafellsíjall
0 98 Icelandite, Sandfellsheidi
0 200 Dacite, Hafrafell
0 135 Dacite, Svinafellsfjall
0 50 Rhyolite, Godafjail
0 261 Rhyolite, Breidamerkurfjall
series (Jakobsson 1979) and of the Coombs trend
(Miyashiro 1978).
As an example of a real transitional series with
peralkalinity appearing at SiO., about 68 wt%, the
trend of the Bouvetoya series (Imsland et al. 1977) is
plotted in Fig. 3. The Öræfajökull rocks are consist-
ently less alkaline than the Bouvetoya series, and
this is especially clear in the case of the silicic rocks
of the two series, a feature that may be related to
different modes of formation. The slightly peralka-
line rhyolites of the Bouvetoya series were probably
derived by fractionation from the more basic rocks
oftheseries (Imslandetal. 1977) whereasmostrhyol-
ites of Öræfajökull are thought to have been formed
JÖKULL 32. ÁR 73