Jökull - 01.12.1979, Síða 66

the Postglacial and they form the active volcanic
zones which intersect Iceland from southwest to
northeast (Fig. 1).
The Postglacial extrusives form distinct
petrological provinces. About 18 volcanic systems
have produced rocks belonging to the tholeiitic
series, and these systems form what can be termed
the axial volcanic rift zone of Iceland (Fig. 1).
Active tectonic fissures and normal faulting with
formation of shallow grabens is characteristic of
these systems.
The tholeiitic systems vary appreciably with
regard to size, the range of rock compositions
extruded and the rate of extrusion. On the
Reykjanes Peninsula (Fig. 6) three types of basaltic
lavas are recognized in the comparatively small
systems, i.e. picrite basalts (oceanites, cf. Table 1,
no. 1) forming small lava shields, olivine tholeiites
forming large lava shields and tholeiitic lavas
extruded from volcanic fissures (cf. Table 1, no. 2).
There is thus a distinct correlation between the
morphology and the chemistry of the lavas.
Petrochemically, two trends are indentifiable,
being a lava shield oceanite — olivine tholeiite
trend and a fissure tholeiite trend.
Studies on the western Reykjanes Peninsula
suggest that observed regular temporal variations
may reflect cyclic volcanic activity such that each
cycle' begins with the formation of oceanites.
Fractional crystallization is recognized as being
important in the development of the rocks, but as is
Fig. 6. The Postglacial
volcanic systems of the
Reykjanes-Langjökull
zone and northernmost
Reykjanes Ridge. Sugges-
ted location of the plate
boundary is shown. Also
shown is the location of
high-temperature thermal
areas and known picritic
lava shields. From Jakobs-
son et al. 1978.
so often found when a detailed study is made, other
processes must also be sought to explain the
compositional variation; for example variations in
degree of partial melting of the source rocks and
volatile transfer or other gravitational processes
working in the magma at shallow level.
Intermediate and acid rocks have not been found
outcropping on the Reykjanes Peninsula except in
the Hengill system.
In the two tholeiitic fissure swarms of the eastern
volcanic zone (Fig. 7) the basalts are tholeiities of a
narrow compositional range. Most of the Veidivötn
basalt lavas carry abundant macrophenocrysts of
plagioclase (An 88), probably cumulated in the
magma at shallow depth by a floating mechanism.
Although the Lakagígar swarm is for the most part
covered by the Vatnajökull glacier, chemical and
petrographic analyses indicate that the subglacial
Grímsvötn caldera also belongs to this volcanic
system. Several very voluminous basalt lavas have
been extruded from these two tholeiitic swarms,
such as the prehistoric Thjórsá B lava (13.5 km3)
and the well-known Lakagígar lava of 1783—1784
(13 km3).
In the northern zone, the tholeiitic Krafla fissure
swarm has been studied in detail from various
points of vjew, mainly in connection with the
present rifting and eruption episode, which has
been in progress since December 1975. It has been
suggested from seismic and ground deformation
evidence, that magma is being fed continously into
64 JÖKULL 29. ÁR