Jökull - 01.12.1983, Page 36

Fig. 1. The volcanic zones in Iceland as they are
disinguished as rift zones and non-rifting zones. A
part of the zone in Central Iceland (marked A)
has, however, apparently been fading out as a rift
zone during the late glacial and postglacial times.
Simplified from lmsland (in press).
Mynd 1. íslensku gosbeltin og skipting þeirra í rekbelti og
reklaus. Hluti gosbeltisins um Mið-ísland (merkt A)
virðist sýna minnkandi rek frá því á seinni hluta ísaldar.
Kortið er einfóldun á korti Imslands (í útgáfu).
system of the world oceans, regarding petrochem-
istry and its relation to volcano-tectonics, has been
reached. Simplified, this general picture of the pet-
rochemistry of the ocean floor depicts primarily
MORB-type tholeiites, produced at the volcanic-
ally active central axis (rift zones) of the Mid-Ocean
Ridges, and less active flanks and older ocean floor
parts, containing a diversity of evolved tholeiites
and non-MORB-type basalts (see e.g. Bryan et al.
1976). This diversity is occasionally simplified and
summarized under the FETI-basalt heading.
In contrast, the general picture of the Icelandic
petrochemistry shows us volcanically active areas,
some of which are rift zones while others are non-
rifting zones (see Fig. 1). The rift zone volcanism in
Iceland produces primitive MORB-type tholeiites
and tholeiites ofweak alkaline enrichment, trending
towards the FETI-basalts. The volcanism of the
non-rifting zones produces primarily basalts of
strong FETI-basalt characteristics with clear alkal-
ine affinities, but generally little -and in most cases
no — primidve tholeiitic rocks. In addition to this
basalt production, both zones produce small vol-
umes of silicic and intermediate rocks, which are
extremely scarce, but occur among the ocean floor
samples. In Iceland this production of silicic and
intermediate rocks takes place in central volcanoes
(for further information on this generalized and
simplified picture of the Icelandic situadon see Sae-
mundsson (1979) and Imsland (in press).
THE CHEMICAL RANGE OF THE
ICELANDIC ROCK SUITES AND
A SAMPLING BIAS
In the rift zones of Iceland, faults, open fissures,
and crater rows, the surface expressions of the rift
tend togroupintoswarms, so-called fissureswarms.
Centrally these may evolve into major volcanoes as
timegoeson (Sigvaldasonetal. 1976), so-called centr-
al volcanoes. This evolution comprises changes in
the endre volcanic features, but in this context the
changes in petrochemistry are of greatest interest.
The fissure swarms, and the scarce but frequently
voluminous volcanism in between fissure swarms,
produce primarily MORB-type rocks i.e. primitive
tholeiites having low incompatible element content,
a high K/Rb, Na/K, and Mg/Mg+Fe ratios and so
on. These are frequently called olivine tholeiites.
Imsland (in press) places the initiation of the cent-
ral volcano stage at the first appearance of qz-norm-
ative rocks on the fissure swarm, as no gap or drastic
change marks its inidation. Afterthis stage is reach-
ed, the rift zone central volcanoes produce, in addit-
ion to the primitive MORB, evolved tholeiites and a
small volume of silicic and intermediate rocks.
These evolved tholeiites are qz-normative, enriched
in the incompatible elements, and have lower K/
Rb, Na/K, and Mg/Mg+Fe ratbs and so on, than
the primidve MORB-type tholeiites. Compositíon-
ally they approach or trend towards the extreme
FETI-basalt characterisdcs.
Those recent rocks, which are atypical for the
oceanic environment and are erupted in the rift
zones of Iceland are thus produced solely in the
central volcanoes, which only cover very small areas
of the entire rift zones. The bulk of the rift zones is
thus covered by the MORB-type rocks. The dredge
sampling method used on the Icelandic rift system
would accordingly give overwhelmingly MORB-
type samples, as it does in the case of the oceanic
spreading axis.
The rocks that form the basis for the concept of
the “Iceland geochemical anomaly” are thus pro-
duced in the central volcanoes ofthe rift zones and
primarily on the non-rifting zones. The causes of the
non-rifting volcanism are discussed by Oskarsson et
34 JÖKULL 33. ÁR