Jökull - 01.12.1983, Síða 35

Biased chemical Range of Icelandic and oceanic Basalt
Analyses: The Result of diíferent sampling Methods
and compositionally selective kinematic Evolution
within Rift Zones
PÁLLIMSLAND
Nordic Volcanological Institute, University of Iceland, 101 Reykjavík
ABSTRACT
Over 50% of the rocks produced in the rift zones of
Iceland are of the primitive MORB-type tholeiites. The
remaining 50% consist of more evolved tholeiites, some
FETI-enriched tholeiitic basalts and a small amount of
intermediate and silicic rocks. The more evolvedpart of this
suite is produced in central volcanoes, while the primitive
part is the product of thefissure swarms in early stages of
evolution. In the non-rifting volcanic zones, no primitive
tholeiiles areproduced, but the bulk of theproduction is of the
FETI-basalt type and enriched in alkalies. The relative
share of intermediate and silicic rocks in these zones is much
greater than in the rift zones, but the total rock production is
much lower. Compared to the ocean ftoor, the Icelandic rift
Zones produce identical rock types but in greater relative
volume. There is a close resemblance between the non-rifting
Zoneproduction and the poorly definedoff-ridge rocks of the
oceans. The Tertiary rocks of Iceland which show low
abundance of the most primitive tholeiites are the flank
products of the main rift zones. This same low abundance
characterizes the ridgeflanks and olderparts of theseafloor.
This harmonizes with the kinematic evolution of the spread-
ing centers and its compositionally selective nature. The idea
behindlhe “Iceland geochemical anomaly” resultsfrom: I)
different data-banks and totally differenl information on
samples of the different data-banks, which result from the
different sampling methods in use on the ocean floor and in
Iceland, and 2) compositionally selective nature of the kine-
matic evolution of the rift zones.
INTRODUCTION
In the literature on the petrochemistry of the
oceanic rocks, one frequently meets with the idea of
an “Iceland geochemical anomaly”. This phrase is
poorly deftned but highly suggestive regarding the
petrochemical diíference between Iceland and the
ocean floor. In the article, this idea will be examin-
ed. The data used is ofthree types: 1) general know-
ledge of the volcanology of Iceland and the evolu-
tion taking place on and within its volcanic zones
(largely summarized by Saemundsson 1979), 2) avail-
able data on the chemistry of the Icelandic rocks
and their distribution in relation to composition
(summarized by Imsland 1978 and inpress), and 3)
for comparison, data from articles summarizing the
petrochemistry of the ocean floor rocks (e.g. Cann
1971 and Hart 1976).
VOLCANO -TECTONICS
AND PETROCHEMISTRY
VERSUS SAMPLING METHODS
The methods used in collecting the rocksamples
from the ocean floor and from Iceland lead to fund-
amentally different background information on the
samples.
In Iceland the samples are hand-picked after
being selected on basis of general geological and
volcanological information. The data-bank on Ice-
landic rock chemistry thus allows the correlation of
the petrochemistry to a diversity ofvolcanic pheno-
mena and other relevant features.
Both the dredge sampling- and drilling methods
used in the case of the ocean floor sampling give
samples without precise geological and volcanolog-
ical information. The chemistry of the ocean floor
rock samples can thus not be correlated to volcano-
tectonic origin in any detail. Only gross correlations
or hypothetical deductions can be made.
On the basis ofinformation gained in this way the
present general ideas on the evolution of the rifting
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JÖKULL 33. ÁR 33