Jökull - 01.12.1983, Page 37

al. (1982) and will not be discussed here, but it is
tirmly stressed that the productivity of the non-rift-
ing volcanism is much less than that of the rift zone
volcanism.
In order to compare the petrology of the Ice-
landic rift zones and the ocean floors, all analysed
tholeiites of the Icelandic rift zones (see Imsland in
press), have been divided into two groups, contain-
ing more and less than 0.2wt.% K,,C), respectively.
The 0.2% value is an arbitrary choice, but a lower
K20 content is a clear indication of a low incomp-
atible element content, a MORB restriction. In this
selection the primitive picritic cumulates ofthe Ice-
landicrift zones (seeJakobssonetal. 1978) containing
over ~ 10 wt.% MgO have been excluded, because
of the strong effect of the picrites on the parameter of
primitiveness used.
These low K,,0 tholeiites are compared in Table
1, on basis of average compositions, to the average
TABLE1
1 2 3
IRZ MORB OFB
sío2 48.56 49.92 49.61
tío2 1.21 1.46 1.43
ai2o3 15.56 16.08 16.01
FeOtot- 10.64 9.38 11.49
MnO 0.18 0.17 0.18
MgO 8.74 7.75 7.84
CaO 12.89 11.21 11.32
Na^O 2.00 2.79 2.76
K,Ö 0.12 0.17 0.22
p2o5 0.11 0.15 0.14
Fe203/Fe0 0.32 0.19
At. Mg/Mg+Fe 0.588 0.597 0.550
No. of samples
in average 96 49 94
Average composition of Icelandic and Oceanic
MORB. 1) IRZ = Icelandic rift zones (Imsland in
press) 2) MORB = Mid ocean ridge basalts (Hart
1976) 3) OFB = Ocean floor basalts (Cann 1971).
Tajla I. Xleðaltalssamsetningar Jyrir hiðJrumsUða ba-
salt aj rekbeltum Islands og úlhajshyggjanna. 1) IRZ =
rekbelti íslands (Imslandíútgáju) 2)X10RB = Miðhajs-
hiyggjabasatt (Hart 1976) 3) OFB = Úthajsbasalt
(Cann 1971).
MORB compositions reported by Cann (1971) and
Hart (1976) for the ocean floor. This selection of the
lcelandic rift zone tholeiites depicts a slighly lower
incompatible element content than the two ocean
floor MORB averages they are compared to, but an
identical primitiveness, as expressed through the
Mg/Mg-f Fe ratio.
The average presented in the table comprises
about halfof the analysed basalt samples of the rift
zones in Iceland which are sampled on basis of
geological knowledge of the sample locations, and
equally represent all geological formations, regard-
less of their volume relations. The volume relations
are overwhelmingly in favour of the MORB-type.
This average thus shows that in terms ofvolume the
Icelandic rift zone products overwhelmingly are of
the MORB-type composition.
The other half of the rift zone rocks are: 1) Iess
primitive, more incompatible element enriched
MORB-tholeiites, which dominate this rest, 2) the
incompatible element enriched relatives of the
MORB, the FETI-basalts and so on, and 3) a small
portion of highly evolved rocks, tholeiitic icelandit-
es, icelandites, dacites and rhyolites.
Various approaches and calculations have been
used in the past to estimate the amount of inter-
mediate and silicic rocks in Iceland. The estimates
obtained range from 3 to 9% of the total volcanic
production. Sigurdsson (1970) found 3% of the total
postglacial production to be of intermediate and
silicic composition and Thorarinsson (1967) found
that only 4% of the intermediate and silicic prod-
ucts in Iceland formed during postglacial time were
produced in the volcanic rift zones. The other 96%
are products of non-rifting volcanism. It is thus
clear that only 0.12% of the rift zone production is
of intermediate and silicic compositions.
If the primitive MORB-type tholeiitic magma is
the primary magma type of the oceanic volcanism
and the parental magma to the tholeiitic rock suites,
then the general increase in rock evolution, through
the iron- and incompatible element enrichment of
the evolved tholeiites and towards the intermediate
and silicic rocks to rhyolites, apparently displays
the entire evolution of the oceanic tholeiitic rock
suits. The relations of the rock chemistry of indivi-
dual samples to geological features, such as fissure
swarms and volcano-tectonics ingeneral, are pract-
ically unknown in the case of the ocean floor. In
Iceland, on the other hand, it has beenshown that
the magma evolution in the rift zones takes place
along with volcano-tectonic evolution (Sigvaldason et
JÖKULL 33. ÁR 35