Jökull - 01.12.1979, Page 60

Fig. 2. Alkaliisilica dia-
gram indicating the
chemical trends of six Ice-
landic volcanic systems,
chosen to represent the
three rock series. Tholeiitic
series: Krafla and Thing-
múli. Transitional alkalic
series: Hekla and Torfa-
jökull. Alkalic series: Vest-
mannaeyjar and Setberg
II. The broken line is the
Hawaiian division line.
above sea level is made up of volcanic rocks
(including near-surface intrusions), only about 10%
being consolidated sediments. These sediments are
mainly interbedded tuffaceous layers of short
transport and moraines and will not be dealt with
here. Metamorphic rocks, conventionally speaking,
are not found except ice-rafted examples. However,
the zeolitic facies of burial metamorphism is
reached in the deepest sections of the Tertiary lava
pile in eastern and southeastern Iceland, where
probably some 1000—1800 m have been eroded
away since the cessation of volcanism in these areas.
THE VOLCANIC SYSTEMS
The presence of rather well-defined volcanic
units, here called volcanic systems, both in the
Tertiary and Pleistocene series, as well as in the
active volcanic zones (Fig. 1), is now generally
acknowledged. A volcanic system is a spatial
grouping of eruption sites, including upper crustal
feeder dykes, active within a relatively short period
of time and with certain limited tectonic,
petrographic and geochemical characteristics. The
term volcanic system may refer to a volcanic fissure
swarm or a central volcano, more commonly to
both where they are associated into one structural
unit.
Usually, a centrally situated complex is built up
in the system, where the discharge of magma is
highest and where a caldera and a high-tem-
perature area may develop. In many volcanic
systems the volcanism exhibits a compositional
zoning. Acidic volcanism is confined to the central
complex, rocks of intermediate composition occupy
a broader zone around the acid center, and only
basalt is erupted in the more distal parts of the
system. All these features are indicative of shallow
magma reservoir(s) under the central area of the
system.
The surface dimensions of volcanic systems in
the active volcanic zones vary between 17X6 km
and 100X18 km. Present data on the volcanic
zones (Fig. 1) indicate that these zones actually
consist of about 26—28 volcanic systems. The
mapped active systems of southwest and south Ice-
land are shown in Figs. 6 and 7.
The division into volcanic systems is of great
importance in understanding the petrogenesis of
Icelandic rocks. The volcanism is considered to
operate within such a closed system for a limited
period of between several hundred thousand to a
million years and to develop a distinct rock suite.
THE THREE ROCK SERIES OF ICELAND
Available data on the petrology of Iceland
indicate that three main rock series have developed
in Iceland, a tholeiitic series, a transitional alkalic
series and an alkalic series.
The tholeiitic series is generally characterized by
a relatively high content of Fe and Ti, and a low
content of A1 and Ca. The content of normative
hypersthene of the basalts is mainly between 10—
19%, and the rocks plot below the division line in
the alkalúsilica diagram of Fig. 2. The tholeiitic
series is made up of the following main rock types:
oceanite, olivine tholeiite, tholeiite, basaltic ice-
58 JÖKULL 29. ÁR