Jökull - 01.12.1983, Síða 38

al. 1976, Jakobsson et al. 1978, Imsland in press).
Fissure swarms evolve into central volcanoes at the
same time as the rocks evolve from primitive tholei-
ites (MORB) alone towards increasing volumes of
qz-normative and evolved tholeiites and further to-
wards icelandites and dacites and finally rhyolites.
Therefore it is concluded, since the Icelandic rift
zone rock suites and the oceanic rock suites do not
differ in types, that the Icelandicrocksuites serve as
a key to the classification of oceanic tholeiites in
general, both regarding chemical evolution and tec-
tonic setting - but not vice versa.
The non-rifting volcanism in Iceland has not
been specifically dealt with here, but in a generaliz-
ed manner it may be compared to the off-ridge and
remote island volcanism in the oceanic environ-
ment.
It may be concluded that, as a whole, Iceland
produces a relatively greater volume of evolved
rocks than the submerged oceanic spreading ridges,
but the Icelandic rift zones do soonly to a very small
degree. The evolved rocks of Iceland and the ocean-
ic environment in general are of the same types. The
difference between Icelandic and ocean floor rock
production is in proportions, not in types. When
refering to the presently active rift zones, the “Ice-
land geochemical anomaly” is the result ofdifferent
sampling methods and lack of background informa-
tion on samples from the ocean floor.
THE CAUSES OF BIASED CHEMICAL
RANGE IN RIFT ZONE PRODUCTS
EXPOSED OFF-RIFT
In the discussion above, reference has been made
to the pattern of volcano-tectonics as well as petro-
chemical evolution of the Quatemary volcanism of
Iceland, where the entire chemical range of the
products of the volcanic rift zones can be studied.
Wood (1978) refers to the “Iceland geochemical ano-
maly”, based on studies of the Tertiary rocks of
Eastern Iceland. This lava pile is composed of the
lateral products of the rift zones. The entire pro-
duction of a rift zone cannot be represented by the
lithospheric segment pushed out of the rift zone,
according to the kinematic evolution of the rift zon-
es. This is demonstrated by Fig. 2, which simplifies
a mathematical model of the rift zone kinematics
presented by Palmason (1973, 1980).
According to the kinematics, the lava pile ofEast-
ern Iceland is all produced in relatively small lateral
VOLCANIC RIFT
Fig. 2. A model diagram of the evolution of a
lithospheric plate, generated in a rift zone. Iso-
chrons and the trajectories of the right half of the
rift zone products are shown. The figure is simplifi-
ed from Palmason (1973) and serves to account, in a
general way, for the formation of the oceanic plate,
including Iceland. It shows how the products of
the central parts of the rift zones subside and drift,
such that they are never exposed on the surface.
Only the lateral products of the rift zones drift out
of the zone on the surface or without subsiding
below levels reached by general later erosion. The
erosion of the Tertiary lava piles of Iceland does
not exceed 2 km and probably represents the deep-
est erosion levels of an undeformed oceanic bot-
tom. The scales of the figure are dependent on the
spreading rate, volcanic production and rift zone
width.
Mynd 2. Líkan-mynd er sýnir þróun jarðskorpu, sem
verður til í rekbelti. Jafnaldurslínur (isochrons) ogfærslu-
ferlar efnisins (transþort trajectories) eru sýnd fyrir htegri
helming rekbeltisins og þeirrar skorþu, sem þar verður til.
Myndin er einfóldun á mynd Guðm. Pálmasonar (1973)
og sýnir á almennan hátt, myndun úthafsjarðskorpu, þar
með talið ísland. Myndin sýnir hvernig efni, sem berst til
yfirborðsins í miðhluta rekbeltisins sekkur og rekur burt án
þess að eiga nokkum tíma móguleika á að Jinnast aftur á
yfirborðinu. Pað sem berst til yfirborðsins hliðhegt í
rekbeltunum rekur út úr beltunum á yfuborðinu eða svo
grunnt að það getur fundist aftur eftir rof. Rofið í tertíera
jarðlagastaflanum á íslandi er undir 2 km og er það
líklega hámarksrof á úthafsbotni Jýrir utan fellingasvteðá.
Skalar myndarinnar eru háðir rekhraða,framleiðni eldvirkn-
innar og breidd rekbeltanna.
fissure swarms and central volcanoes which were
driven out of the rift zone by more efíicient and
36 JÖKULL 33. ÁR