Jökull - 01.12.1979, Page 68

6
5
0 -------------------1--------1--------1--------1-------l.
45 46 47 48 49 50 51
SÍO2 wt. %
O Alkali olivine basalt
A Transitonal basalt
• Tholeiite
■4* Olivine tholeiite
+ Picrite basalt
52 53 54
55
Fig. 8. Alkali:silica dia-
gram of Postglacial basalts
from the Reykjanes-Lang-
jökull zone (Fig. 6) and the
eastern volcanic zone (Fig.
7).
belonging to the same system (trend) as the
intermediate and acid rocks of Ffekla itself. The
very high proportion ( ~ 40 per cent by vol.) of
intermediate rocks (cf. Table 1, no. 7) produced in
this system is exceptional for Postglacial volcanism.
Acid tephras and lavas are only erupted from the
top crater of Hekla. It has been found that there is a
positive correlation of the Si02 — content of the
first melt erupted with the volume of tephras and
lavas erupted each time, as well as with the length
of the preceeding quiescent period. Opinions have
differed as to the origin of the Hekla rock suite.
Fractional crystallization of a basaltic parent,
successive partial melting of subsided basalts at
high water pressure and assimilation of acid rocks
at depth have particularly been discussed as
possible models.
The Torfajökull volcanic system is still
somewhat loosely defined. Rhyolitic rocks of Upper
Pleistocene age cover some 400 km2 and the origin
of these has been a matter of dispute.
Measurements of Sr-isotope ratios in this area
indicate that the evolved rocks of the Torfajökull
complex cannot have been derived by fractional
crystallization from the basalts. ___
Probably three alkalic volcanic systems are
active in the Snaefellsnes flank zone, and one, the
Vestmannaeyjar, in the eastern flank zone. Only
alkali olivine basalts (cf. Table 1, no. 9) and
hawaiites (cf. Table 1, no. 10) have been produced
Fig. 9. Below: Diagrammati-
cal representation of the dis-
tribution of K20 in the axial
rift tholeiites with distance
along the ridge axis in the
Iceland area. Columns with
arrows indicate the presence
of Recent central volcanoes
with evolved rock com-
positions.
Above: Histograms indicating
discharge of volcanic rocks in
Postglacial time across Ice-
land. Oblique lines indicate
tholeiites, dots alkalic basalts
and black intermediate to
acid rocks. The inset map
with the solid line shows the
location of the profile.
66 JÖKULL 29. ÁR